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Bosch GWH 635 ES BoschProTankless™ GWH 635 ES Gas Tankless Water Heater Troubleshooting Guide

Please note: If you are unable to perform the tasks listed below or need additional assistance, please contact your original installer/licensed gas technician. For indoor installation only.

Troubleshoot With No Error Codes:

Burners Do Not Ignite When Hot Water Is Turned On?

1. No power to heater/ display is blank. Verify power to outlet. (120VAC 60Hz properly grounded circuit required) Consult installation manual to ensure a proper electrical connection. Verify the heater on/off switch is in the on position Verify the fuses in the control board are good. Consult electrical diagram in the installation manual for fuse locations and See: TWH G2-24 Replacing control unit to access fuses.
2. Incoming cold water feed connected to wrong side of heater. Make sure cold water inlet connection is plumbed to the right side of heater when facing unit.
3. Water flow rate at hot water tap is too low: A minimum of 0.8 gallon/minute (3 l/m) is required to activate the heater.
4. Verify inlet filter screen is clean and clear of debris. For location of inlet filter screen, See: TWH G2-02 Service Bulletin.
5. Inspect the water path outside of the heater for obstructions. Make sure all outlets (i.e. showerheads, faucet aerators and whole house filters) are clear of debris.
6. Crossover in plumbing: The heater activates when water flow through the unit is at or above the required minimum of 0.8 gallon/minute (3 l/m). A crossover in the plumbing creates back pressure on the water flowing through the heater. Therefore, a higher flow rate than normal is needed to force the heater to activate. To confirm a plumbing crossover, shut off the cold water supply feed to the water heater. Then open all of the hot water taps served by the heater. Wait 10 minutes and check for water flow at taps. There should be no water flowing. Any continuous flow of water, small or large, confirms a crossover and needs to be corrected immediately. Failing single lever faucets and mixing valves are common culprits for plumbing crossovers.
7. Loose wire connection. With the on/off switch turned to off position and the power supply cord unplugged, remove the unit's front cover. Check all wire connections, paying close attention to all connections associated between the water valve, control unit and set of electrodes. See parts diagram in manual for location of these parts see: 635 Manual.
   

Water Is Too Hot?

1. Selected temperature on the unit is set too high. See: TWH G2-02 Overcoming fluctuating temperatures.
2. Restrictions in the water path. This slows the water flow through the heater forcing it to spend too much time over the burner flame. Make sure all outlets (i.e. showerheads, faucet aerators and whole house filters) are clear of debris. Also, the heater's inlet filter screen should be inspected and cleaned. See: TWH G2-02 Service Bulletin.
3. Wrong gas type. Please ensure the gas type indicated in the rating sticker on the unit's right side coincides with the gas type you are using.
4. Inlet water is preheated. This model is designed for cold water feed only. For solar preheated applications, use of model 125BS is recommended. See: 125BS Manual.
5. The inlet cold water is greater than 70°F due to geographic location. Avoid restrictive outlets. Clean all showerheads and faucet aerators. Possibly upgrade to higher flowing gallon per minute aerators and showerheads.
6. If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing the water to reach boiling temperatures. Run a descaling solution of 50% water and 50% vinegar solution through the heater. Repeat process until heater is free of scale. Use of a water softener is suggested.
7. Temperature sensor misaligned. Ensure both temperature sensors are making contact and firmly mounted on their respective cold and hot water pipes. See: 635 Parts Diagram.
   

Water Is Not Hot Enough?

1. Selected temperature on the unit is set too low.
   See: TWH G2-02 Overcoming fluctuating temperatures.
2. Verify inlet filter screen is clean and clear of debris.
   For location of inlet filter screen See: TWH G2-02 Service Bulletin.
3. Wrong gas type.
   Please ensure the gas type indicated in the rating plate sticker on the right side of unit coincides with the gas type you are using.
4. Obstruction in the gas path.
   Check inlet gas particle screen at gas inlet connection on bottom of unit.
5. Lack of gas pressure.
   Ensure gas pressure is in accordance with specifications in the installation manual. A gas pressure reading is needed to proceed further. Contact your original installer or a local certified gas technician to obtain this reading. See: TWH G2-03 Checking gas pressure.
6. Cold water is mixing into the hot water lines (plumbing crossover).
   A plumbing crossover can unintentionally mix cold water with the hot water leaving the heater. The end result is a cooler temperature than desired reaching the outlet. To confirm a plumbing crossover, shut off the cold water supply feed to the water heater. Then open all of the hot water taps served by the heater. Wait 10 minutes and check all taps for water flow. There should be no water flowing. Any continuous flow of water, small or large, confirms a crossover and needs to be corrected immediately. Failing single lever faucets and mixing valves are common culprits for plumbing crossovers.
7. Temperature sensor misaligned.
   Ensure both temperature sensors are making contact and firmly mounted on their respective cold and hot water pipes. See: 635 Parts Diagram.
   

Low Water Flow/Pressure?

1. Too many hot water applications are being used simultaneously or too much flow is demanded. The water heater will effectively support two 2.0-2.5 gpm shower heads simultaneously or multiple sink applications. Greater draws will result in increased pressure drop and reduced flow at taps.
2. BTU input is too low due to inadequate gas line sizing or lack of gas pressure. Either one of these deficiencies will cause the water heater to close its motorized water valve, reducing the hot water flow rate to effectively reach the selected output temperature. See: TWH G2-03 Checking gas pressure.
3. Selected temperature on the unit is set to high for the demanded flow rate. The water heater will close its motorized water valve, reducing the hot water flow rate to effectively reach the selected output temperature. Lowering the selected temperature will allow the motorized water valve to open up for increased water flow rate.
4. Cold water inlet filter screen is dirty: The inlet filter screen should be inspected and cleaned. See: TWH G2- Locating and cleaning inlet filter screen.
5. Inspect the water path outside of the heater for obstructions. Make sure all outlets (i.e. showerheads, faucet aerators and whole house filters) are clear of debris.
6. Motorized water valve is stuck. See: TWH G2-11 Unlocking stuck water valve.

Hot Water Temperature Fluctuates At Tap; Unit Deactivates?

1. Hot water is very hot out of the tap, requiring a lot of cold water to be added with it in order to attain a useable hot water temperature. The addition of too much cold will overpower hot water demand from the tankless water heater. This slows the flow within the tankless water heater, decreasing it below activation point, which shuts off the burners. The end result is nothing but cold water coming out of the outlet. See: TWH G2-02 Overcoming fluctuating temperatures.
2. Unbalanced pressure in water lines.
   Any restriction in the water heater, such as a clogged inlet filter screen, can result in unequal pressures between the cold and hot water lines. In such cases, when mixing in the higher pressure cold water at the tap, the lower pressure hot water can be overpowered. This can cause the hot water flow rate to fall below the minimum rate required causing the heater to deactivate. Verify inlet filter screen is clean and clear of debris. See: TWH G2-02 Service Bulletin.
3. Heater deactivated by Temperature balancing valves.
   If the outlet water temperature is set too high, the heater can produce temperatures that are too hot. A temperature balance shower valve can automatically mix in cold water to reduce such hot water temperature. In the event of any temperature instability at a fixture using a temperature balancing valve, refer to the valve manufacturer for instructions on internal adjustment setting. An adjustment should be made to minimize the amount of cold water the valve is adding. Additionally, the temperature setting on the heater can be lowered to prohibit the temperature balance valve from mixing in too much cold.
4. Inlet water pressure is erratic due to fluctuating supply water pressure.
   For installation on a private well system with the use of a pressure tank, the lowest pressure range setting recommended is 30-50 psi (2.07-3.45 bar). The use of a pressure reducing/regulating valve directly after the pressure tank is an effective way to maintain constant water pressure to the water heater. Watts brand 25AUB- ¾" or N35B-¾" pressure reducing/regulating valves or equivalent are suggested.
   

Noisy Burner/Heater During Permutation?

1. Bad front cover seal. Make sure cover is securely fastened. Improper seal between the heater chassis and the front cover could result in noise.
2. Improper venting. Venting that is unsealed, the wrong material, too big in diameter or too long in run will result in unstable burner flames causing noise. Ensure venting is proper and in accordance with specifications in the installation manual.
3. Lack of adequate combustion air. Drawing combustion air from a room area of inadequate size will result in unstable burner flames causing noise. Improper piping of combustion air to the outside or sufficient space will result in unstable burner flames causing noise. Ensure adequate combustion air is provided to the unit in accordance with specifications in the installation manual.
4. Cross contamination. Ensure intake and exhaust terminations maintain required clearances stated in the manual. Cross contamination between intake and exhaust may cause unstable burner and noise.
5. Lack of gas pressure. Inadequate gas pressure will cause the fuel-to-air mixture(CO2) to be out of adjustment. This will result in unstable burner and noise. Ensure gas pressure is in accordance with specifications in the installation manual. A gas pressure reading is needed to proceed further. Contact your original installer or a local certified gas technician to obtain this reading. See: TWH G2-03 Checking gas pressure.
6. CO2 range out of adjustment. Verify CO2 reading. CO2 adjustments can only be done by a certified gas technician with a calibrated Combustion Gas Analyzer. For proper CO2 ranges see: TWH G2-12 Measuring and adjusting CO2.
   

Troubleshooting with Error Codes:

Please note: If there is a problem with the installation, venting or operation of the unit, the heater will communicate the source by these error codes.

A2 - Fault in the overheat cutoff fuse (white overheat wire/fuse that wraps around the heat exchanger).

1. Disconnect plug connector between fuse wire and control unit. With a multimeter, check continuity (resistance) through fuse to determine if the fuse is bad. The circuit should read closed.
2. If fuse is bad, determine cause of fuse failure. Check for leakage of flue gases on the heat exchanger. Inspect black rubber seals located on top and bottom of heat exchanger. Inspect seal around viewing window on heat exchanger. Call Technical Support (800) 642-3111 for fuse replacement and to report any leakage areas.
3. If fuse is bad, ensure venting is in accordance with specifications in the installation manual. Improper venting, such as long runs, too many elbows, blocked venting or shared venting with another appliance can contribute to fuse failure. Correct venting and call Technical Support (800) 642-3111 to order a replacement fuse.
4. If fuse is okay, check plug connector between fuse and control unit for bent prongs and ensure connection is tight.
5. If fuse is okay, unplug power supply cord on water heater and check two main plug connections on control unit. Loosen two screws on lower half of control unit to remove access panel. Disconnect and reseat two main plug connections to control unit and try resetting.
6. If the fuse is okay, the unit resets but continually faults back to A2 with water flow, the fuse wire may be pinched but not broken. Call Technical Support (800) 642-3111 for fuse replacement.
   

A7 - Hot Water Temperature Sensor fault

1. Locate temperature sensor on hot water pipe. Ensure the red lead wires to the sensor are connected tightly and sensor is firmly clipped on hot water pipe.
2. Check temperature sensor and contacts for corrosion. Clean contacts with light sandpaper or an eraser. If corroded badly, contact Technical Support (800) 642-3111 for replacement temperature sensor.
3. Sensor may trip if heater is exposed to near-freezing conditions (36°F or less). Heater must be protected from freezing temperatures. Temperatures below freezing will cause damage not covered by the warranty.
4. Sensor may trip if heater is producing very hot temperatures above the selected outlet temperature. Check for restrictions in the water path. This slows the water flow through the heater forcing it to spend too much time over the burner flame. Make sure all outlets (i.e. showerheads, faucet aerators and whole house filters) are clear of debris. Also, the heater's inlet filter screen should be inspected and cleaned. Cold water inlet screen dirty. See: TWH G2-02 Service Bulletin.
5. If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing the water to reach boiling temperatures and the sensor to trip. Run a descaling solution of 50% water and 50% vinegar solution through the heater. Repeat process until heater is free of scale. Use of a water softener is suggested.
6. Sensor may be defective. Temperature sensors are the same on both the hot and cold water pipe. Turn off power to the water heater and disconnect wires to sensors. Swap sensors only. Do not swap wires, just reconnect to new sensors. If the error code changes to E2, then the sensor is defective. Call Technical Support (800) 642-3111 for replacement sensor.
   

A9 - Hot water temperature sensor not sensing correct selected temperature.

This error code may flash on display while heater continues to run or it may shut off the heater requiring resetting of the error code.

1. Check that the temperature sensor is firmly connected to the hot water pipe. The sensor should be connected to the middle of the horizontal section of the hot water pipe.
2. Heater not able to hit selected temperature due to high inlet water temperatures at low flow rates. Verify the heater is being fed cold water only. Do not feed preheated water to this unit. Increase flow rates whenever possible. Check for obstructions and make sure all outlets (i.e. showerheads, faucet aerators and whole house filters) are clear of debris. Also, the heater's inlet filter screen should be inspected and cleaned. Cold water inlet screen dirty. See: TWH G2-02 Service Bulletin.
3. Blocked or improper venting. Failure to vent in accordance with the specifications in the installation manual or an obstruction in the vent path may cause improper combustion, inhibiting the heater's ability to reach selected temperature. Also, verify back draft flapper inside flue gas collector collar opens and closes.
4. Low gas pressure inhibiting the unit from reaching selected temperature. The sensor is not reading expected temperatures. Ensure gas pressure is in accordance with specifications in the installation manual. A gas pressure reading is needed to proceed further. Contact your original installer or a local certified gas technician to obtain this reading. TWH G2-03 Checking gas pressure.
5. Supply voltage too low. If voltage to the unit is low, the fan may not achieve the appropriate speed for proper combustion to reach selected temperature. Verify electrical power supply is a properly grounded 120V AC 60Hz circuit.
6. Defective control unit. Control unit may not be accepting signal from temperature sensor. Call Technical Support (800) 642-3111for further instructions.
   

C1 - Fan rotation too low at start-up.

1. Loose wire connection. Verify wire plug connector located in the rear of exhaust fan is firmly connected. Unplug power supply cord on water heater and check two main plug connections on control unit. Loosen two screws on lower half of control unit to remove access panel. Disconnect and reseat two main plug connections to control unit and try resetting.
2. Supply voltage too low or fluctuating. Fan may not be able to achieve appropriate speed if voltage is not correct. Verify electrical power supply is a properly grounded 120V AC 60Hz circuit. Ensure other appliances plugged into the same outlet are not causing voltage fluctuations. Verify unit is not plugged into an extension cord.
3. Venting is improper or blocked. This could inhibit the fan's ability to rotate. Verify venting is in accordance with requirements in the installation manual. Inspect the entire vent path for obstructions. Verify back draft flapper inside flue gas collector collar is opens and closes.
4. Low gas pressure. The fan may alter its rotational speed to compensate for lack of gas to ensure proper combustion. Ensure gas pressure is in accordance with specifications in the installation manual. A gas pressure reading is needed to proceed further. Contact your original installer or a local certified gas technician to obtain this reading. See: TWH G2-03 Checking gas pressure.
5. Defective control unit. Control unit not sending proper signal to the fan. Call Technical Support (800) 642-3111for further instructions.
   

C6 - Fan rotation too low during operation.

1. Loose wire connection. Verify wire plug connector located in the rear of exhaust fan is firmly connected. Unplug power supply cord on water heater and check two main plug connections on control unit. Loosen two screws on lower half of control unit to remove access panel. Disconnect and reseat two main plug connections to control unit and try resetting.
2. Supply voltage too low or fluctuating. Fan may not be able to achieve appropriate speed if voltage is not correct. Verify electrical power supply is a properly grounded 120V AC 60Hz circuit. Ensure other appliances plugged into the same outlet are not causing voltage fluctuations. Verify unit is not plugged into an extension cord.
3. Venting is improper or blocked. This could inhibit the fan's ability to rotate. Verify venting is in accordance with requirements in the installation manual. Inspect the entire vent path for obstructions. Verify back draft flapper inside flue gas collector collar opens and closes.
4. Cross contamination. Ensure intake and exhaust terminations maintain the required clearances stated in the manual. Cross contamination between intake and exhaust may cause the fan to alter its rotational speed.
5. Low gas pressure. The fan may alter its rotational speed to compensate for lack of gas to ensure proper combustion. Ensure gas pressure is in accordance with specifications in the installation manual. A gas pressure reading is needed to proceed further. Contact your original installer or a local certified gas technician to obtain this reading. See: TWH G2-03 Checking gas pressure
6. Defective control unit. Control unit not sending proper signal to the fan. Call Technical Support (800) 642-3111 for further instructions.

C7 - No rotational speed sensor signal from fan.

1. Loose wire connection. Verify wire plug connector located in the rear of exhaust fan is firmly connected. Unplug power supply cord on water heater and check two main plug connections on control unit. Loosen two screws on lower half of control unit to remove access panel. Disconnect and reseat two main plug connections to control unit and try resetting.
2. Supply voltage too low or fluctuating. Fan may not be getting proper signal if voltage is not correct. Verify electrical power supply is a properly grounded 120V AC 60Hz circuit. Ensure other appliances plugged into the same outlet are not causing voltage fluctuations. Verify unit is not plugged into an extension cord.
3. Damaged electrical components in fan. Call Technical Support (800) 642-3111 for further instructions.
   Defective control unit. Control unit not sending proper signal to the fan. Call Technical Support (800) 642-3111 for further instructions.

CA - Water flow signal over specified maximum value.

1. Loose wire connection. Verify wire connections between control unit and water valve are firmly connected. Unplug power supply cord on water heater and check two main plug connections on control unit. Loosen two screws on lower half of control unit to remove access panel. Disconnect and reseat two main plug connections to control unit and try resetting.
2. Excessive water pressure and flow. Ensure water pressure is less than 150psi and flow rate is below 10 gallons per minute.

E1 - Over-temperature detected by temperature sensor on hot water pipe.

1. This water heater is designed for domestic hot water with a cold water feed only. Recirculation or preheated water applications are not allowed.
2. Temperature sensor on hot water pipe in wrong position or not properly connected. Sensor should be firmly connected on the middle of the horizontal section of the hot water pipe to obtain an accurate reading.
3. Defective temperature sensor on hot water pipe. Temperature sensors are the same on both the hot and cold water pipe. Turn off power to the water heater and disconnect wires to sensors. Swap sensors only. Do not swap wires, just reconnect to new sensors. If the error code changes to A7, then the sensor is defective. Call Technical Support (800) 642-3111 for a replacement sensor.
4. Restriction in the water path. Restrictions in the water path can slow the water flow through the heater forcing it to spend too much time over the burner flame. Make sure all outlets (i.e. showerheads, faucet aerators and whole house filters) are clear of debris. Also, the heater's inlet filter screen should be inspected and cleaned. Cold water inlet screen dirty. See: TWH G2-02 Service Bulletin.
5. If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing the water to reach boiling temperatures and the sensor to trip. Run a descaling solution of 50% water and 50% vinegar solution through the heater. Repeat process until heater is free of scale. Use of a water softener is suggested.

E2 - Cold water temperature sensor fault.

1. Locate temperature sensor on cold water pipe. Ensure the red lead wires to the sensor are connected tightly and sensor is firmly clipped on cold water pipe.
2. Check temperature sensor and contacts for corrosion. Clean contacts with light sandpaper or an eraser. If corroded badly, call Technical Support (800) 642-3111 for replacement temperature sensor.
3. Ambient room temperature at water heater location below 36°F. The temperature sensor will fault on E2 if it detects temperatures below 36°F. The water heater must be protected from freezing temperatures. Temperatures below freezing will cause damage not covered by the warranty.
4. Sensor may be defective. Temperature sensors are the same on both the hot and cold water pipe. Turn off power to the water heater and disconnect wires to sensors. Swap sensors only. Do not swap wires, just reconnect to new sensors. If the error code changes to A7, indicating a hot water sensor fault, then the sensor is defective. Call technical support (800) 642-3111 for replacement sensor.

E9 - Temperature limiter open circuit.

1. Overheat sensor (ECO) located on upper right side of the heat exchanger has tripped. This error should reset once overheat sensor detects cooler temperatures. This water heater is designed for domestic hot water with a cold water feed only. Recirculation or preheated water applications are not allowed.
2. Loose wire connections. Ensure lead wires to the overheat sensor are connected tightly.
3. Dirty or corroded sensor contact. Check overheat sensor contacts where wires connect for corrosion. Clean contacts with light sandpaper or an eraser. If corroded badly, call Technical Support (800) 641-3111 for replacement overheat sensor.
4. Blocked or improper venting. Failure to vent in accordance with the specifications in the installation manual or an obstruction in the vent path can cause an overheat situation. Also, verify back draft flapper inside flue gas collector collar opens and closes.
5. Restriction in the water path. Restrictions in the water path can slow the water flow through the heater forcing it to spend too much time over the burner flame and overheat. Make sure all outlets (i.e. showerheads, faucet aerators and whole house filters) are clear of debris. Also, the heater's inlet filter screen should be inspected and cleaned. Cold water inlet screen dirty. See: TWH G2-02 Service Bulletin.
6. If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing the water to reach boiling temperatures and the sensor to trip. Run a descaling solution of 50% water and 50% vinegar solution through the heater. Repeat process until heater is free of scale. Use of a water softener is suggested.
7. Overheat sensor (ECO) may be defective. Unplug power supply cord to the water heater. Open a hot water tap for several minutes to allow cold water to pass through heat exchanger. Close hot water tap and disconnect lead wires to overheat sensor. Using a multimeter, check continuity through overheat sensor contacts. If circuit reads open, call Technical Support (800) 642-3111 for a replacement overheat sensor.

EA - No flame ionization detected with water flow.

1. Verify all gas manual shut off valves are open in the gas pathway.
2. Wrong gas type. Verify gas type indicated in rating sticker located on the heater’s right-hand side, coincides with the gas type you are using. NG is a natural gas unit and LP is for liquid propane.
3. Air in the gas line. Reset error code and open a water tap to cycle the heater in an effort to purge air. Cycling hot water tap on and off multiple times may be necessary. If heater still faults with EA error code, have a licensed gas technician properly purge air out of the gas line leading to the water heater.
4. Observe inside the viewing window of the heat exchanger for presence of spark when a hot water tap is opened. If no spark is present, verify thick yellow wires leading from control unit are securely connected to the bottom of the electrode set located at the bottom right hand side of the heat exchanger.
   Observe inside the viewing window of the heat exchanger when a hot water tap is opened. Confirm if flame is present prior to the water heater faulting on EA error code. If flame is present, the flame ionization rod may be dirty or have a loose wire connection. Verify thin yellow wire leading from control unit is securely connected to the bottom of the set of electrodes located at the bottom right hand side of the heat exchanger. Failure to vent the water heater in accordance with the specifications in the installation manual may dirty/damage the flame ionization rod. Contact Technical Support (800) 642-3111 to see if replacement is necessary.
5. Low gas pressure. Ensure gas pressure is in accordance with specifications in the installation manual. A gas pressure reading is needed to proceed further. Contact your original installer or a local certified gas technician to obtain this reading. See: TWH G2-03 Checking gas pressure.
6. Gas valve or flame ionization rod may be damaged. Contact Technical Support (800) 642-3111 for further instruction.

EC - Ionization failure while burning.  This error code indicates the heater is not sensing good flame quality.

1. Wrong gas type. Verify gas type indicated in rating sticker located on the heater’s right-hand side, coincides with the gas type you are using. NG is a natural gas unit and LP is for liquid propane.
2. Loose connection to the flame ionization rod. Verify that the thinner yellow leading from the control unit is securely connected to the set of electrodes located at the bottom right-hand side of the heat exchanger.
3. Flame rod is dirty or damaged. Failure to vent the water heater in accordance with the specifications in the installation manual may dirty/damage the flame ionization rod. Contact Technical Support (800) 642-3111 to see if replacement is necessary.
4. Low gas pressure. Inadequate gas pressure will put the fuel-to-air mixture (CO2) out of adjustment. This will result in an unstable burner and lifting flames. Ensure gas pressure is in accordance with specifications in the installation manual. A gas pressure reading is needed to proceed further. Contact your original installer or a local certified gas technician to obtain this reading. See: TWH G2-03 Checking gas pressure.
5. Air-to-fuel mixture (CO2) is out of adjustment. Verify CO2 reading. CO2 adjustments can only be done by a certified gas technician with a calibrated Combustion Gas Analyzer. For proper CO2 ranges. See: TWH G2-12 Measuring and adjusting CO2.
6. Gas valve or flame ionization rod may be damaged. Contact Technical Support (800) 642-3111 for further instruction.

EO - Internal software/hardware failure.

1. Loose wire connections. Verify all wire connections inside the heater are connected tightly. Unplug power supply cord on water heater to check two large white plug connections on control unit. Loosen two screws on lower half of control unit to remove access panel. Disconnect and reseat two main plug connections to control unit and try resetting.
2. Crossed signal inside control unit. Hitting the wrong combination of buttons on the control unit can create confusion among the microprocessors inside. In this case, the error code should not happen more than once or twice. Turn off the water heater. Turn water heater back on and try resetting error code.
3. Control unit may be damaged. Contact Technical Support (800) 642-3111 for further instruction.

F7 - Ionization error at standby.

1. Loose connection to the flame ionization rod. Verify that the thinner yellow wire leading from the control unit is securely connected to the set of electrodes located at the bottom right-hand side of the heat exchanger.
2. Flame ionization rod or control unit may be damaged. Contact Technical Support (800) 642-3111 for further instruction.

FA - Gas leakage error. Gas valve circuit not closing properly.

1. Loose wire connection on the gas valve. Verify wire connection plug from the control unit to the gas valve is firmly connected. Unplug power supply cord on water heater and check two main plug connections on control unit as well. Loosen two screws on lower half of control unit to remove access panel. Disconnect and reseat two main plug connections to control unit and try resetting.
2. Control unit not sending proper signal to gas valve. Flow water out of a hot water tap above the minimum activation point of .8GPM. Measure voltage at the gas valve wire plug connection. The connection should measure 24VDC between the left pair of wires and 24VDC between the right pair of wires when the unit is operating. If voltage is not proper, contact Technical Support for further instruction.
3. Gas valve may be damaged. Contact Technical Support (800) 642-3111 for further instruction.

8E - Electronic noise in the control unit.

1. Improper resetting of other error codes. Using the on/off switch to reset error codes can interrupt signal between microprocessors inside control unit and show 8E on the display. Use the reset button (?) to reset any error codes.
2. Control unit may be damaged. Call Technical Support (800) 642-3111 for further instruction.

Information Source: Bosch Water Heating