bosch tankless water heater error codes

bosch tankless water heater error codes

Is your Bosch tankless water heater displaying an error code? In some cases you may be able to fix the problem on your own and save yourself a service call. Before calling a professional check the error code, if you’re comfortable doing some troubleshooting, you’ll at least have an idea of the issue, and you may even be able to get your tankless up and running again!

When an error message displays on your Bosch tankless water heater its an indication that the unit requires attention. Although you can hit the reset button and clear the error, in many cases, the error will trigger again. Tankless water heaters are expensive and efficient appliances that are designed to last over 20 years. If you take care of your Bosch unit, it’ll provide you with a long service life.

Error Codes for Bosch Tankless Water Heaters


Important: Keep in mind that your best resource is always your owners manual. It’ll have the information needed for your specific Bosch tankless model.

A licensed professional is your best choice to repair your tankless. In addition, if your appliance is under warranty, you run the risk of voiding the warranty if you do the work yourself.

Error Code A2

Flue Gas Limiter

If your Bosch tankless water heater displays an Error Code A2 there was a fault in the flue gas limiter. This is an indication that the unit may be overheating and the temperature within the cabinet has reached 230°F or above. We highly recommend contacting a professional technician who’s licensed and experienced in working with Bosch units. 

  • Check the continuity of the flue gas limiter. Repair or replace as needed
  • Check for flue gas leakage around the heat exchanger’s top and bottom seals. You’ll need a mirror to check in the rear of the heat exchanger
  • Ensure the flue gas limiter connector and spade connections are securely in place
  • Unplug and inspect the wiring harness connections on the control board

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Error Code A3

Wire Harness

An Error Code A3 indicates that there’s an issue with the wire harness.

  • Inspect the wire harness and connections
  • Measure the resistance on the wire harness. It should be between 115 and 120 ohms
  • Contact a licensed technician

Error Code A4

Backflow Temperature Sensor

An Error Code A4 indicates that the backflow temperature sensor has either been disconnected or there’s a short circuit.

  • Inspect the backflow temperature sensor for proper installation
  • Replace the exhaust temperature sensor
  • Contact a licensed technician

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Error Code A7

Outlet Temperature Sensor

This error code indicates that there’s an issue with the outlet temperature sensor. There are three issues that can cause an Error Code A7: The Bosch tankless needs to be descaled; the water is below 36°F; or the water is above 210°F (which could be caused by lime scale build-up on the heat exchanger).

  • Inspect the hot water temperature sensor wiring. It should be free of corrosion, in good condition, and fit snuggly
  • You may need to measure the sensor’s resistance readings. Check your owners manual for details on your specific unit
  • Replace the sensor if needed
  • If you suspect the error code was due to a temperature reading above 210°F. Flush the tankless unit to remove scale build-up
  • If you still see an A6 error reading after flushing, you should contact a licensed technician  
  • If the error code was triggered due to the water temperature dropping below 36°F you run the risk of your tankless unit freezing. Take the appropriate steps to prevent your unit from freezing

Error Code A9

Outlet Temperature Sensor

An Error Code A9 is an indication the outlet temperature sensor is not sensing the expected temperature as the hot water leaves the unit. The A9 Error Code is a status message, not a typical error code, so you’ll see it flashing on the display.

  • Check that the outlet temperature sensor is properly connected to the hot water pipe. The sensor must be attached to a vertical section of the hot water pipe
  • Check the gas pressure. If the gas pressure is low, it may prevent the unit from reaching the necessary temperature
  • Check the supply voltage. It must be a grounded 120V connection
  • Contact a licensed technician for further troubleshooting

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Error Code C6

Slow Spinning Fan

A Bosch Error Code C6 is an indication that the primary fan is spinning too slow. 

  • Reset the error code and disconnect the tankless from the power supply
  • Inspect the fan’s wiring. Check that the fan is correctly wired and that there are no loose wires 
  • Check the voltage to the unit
  • Inspect the venting. Long vents, excessive elbows through the venting system, or a blockage can all cause an Error Code C6
  • Another venting cause could be cross-contamination between the air intake vents and the exhaust
  • Contact a licensed technician for further troubleshooting

Error Code C7

Fan Rotational Speed Sensor

An Error Code C7 indicates that the primary fan rotational speed sensor is not detecting movement.

  • Unplug the unit
  • Inspect the wire connections located on the rear of the primary fan, as well as the connectors to the control board
  • Check that the unit’s supply voltage is 120V and its grounded properly
  • If necessary, replace the sensor, fan, or control unit
  • Contact licensed technician

Error Code CA

Excessive Water Flow

An Error Code CA indicates that your Bosch tankless water heater has a water flow over 10 GPM (gallons per minute).

  • Unplug the unit
  • Inspect the water valve wiring
  • Inspect the PCB wiring
  • Ensure the water pressure is less than 150 psi
  • Ensure the flow rate is below 10 GPM

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Error Code E1

Extremely High Temperature

If you see an Error Code E1 it means that the outlet temperature sensor reading is above 185°F.

  • Inspect the hot water temperature sensor to ensure the wire connection is attached correctly to the hot water pipe and it is in good repair
  • Flush the unit to remove scale build up
  • Contact a licensed technician for further troubleshooting

Error Code E2

Cold Water Temperature Sensor

This is an indication that there’s a problem with the inlet water temperature sensor. An Error Code E2 occurs when the incoming water is below 36°F, or the cold water temperature sensor or water valve are faulty.

  • Inspect the cold water temperature sensor wiring 
  • Measure the resistance to the cold water temperature sensor. This should give you an indication if the sensor or the water valve are faulty and need to be replaced. See your owners manual for the sensor’s location and recommended range for your specific unit
  • If the incoming water is below 36°F, take steps to protect your tankless from freezing

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Error Code E3

Exhaust Temperature Sensor

An Error Code E3 indicates a fault with the exhaust temperature sensor on a condensing tankless water heater. If the temperature is above 194°F the unit will automatically shut itself down and lock for safety. 

  • Inspect both the heat exchanger and condensing heat exchanger to ensure they’re clean and properly working
  • Lower the maximum power
  • Lower the water temperature setting
  • We highly recommend contacting a licensed technician
    to troubleshoot this error code

Error Code E4

Backflow Temperature Sensor

An Error Code E4 means that the backflow temperature sensor has detected a temperature above 309°F. The tankless unit will automatically close down the burner will lock for safety.

  • Check the inlet and outlet vent pipes. Ensure there are no blockages and they meet the specifications (check your owners manual for the specs on your unit)
  • Check the wiring on the secondary fan and ensure it is properly operating
  • We highly recommend contacting a licensed technician to troubleshoot this error code

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Error Code E9

Overheat Sensor

An Error Code E9 is an indication that the overheat sensor (ECO) has an open circuit.

  • Unplug the unit from the outlet
  • Check that the wiring to the overheat sensor is securely in place, and that its clean and in good working condition
  • Check the venting for blockages and that it meets the specification requirements
  • It may be necessary to flush your unit to remove scale build-up 

Error Code EA

No Flame Ionization Detected

An Error Code EA is an indication that there’s no flame ionization detected when the water is flowing.

  • Verify that the unit is receiving gas
  • Inspect the flame color after a hot water tap has been opened. The flame should be stable blue. If the flame is yellow or unstable, check the gas pressure, CO2 readings, and gas type
  • Remove air within the system by resetting the error code and opening and closing a hot water tap several times. It may be necessary to contact a professional technician to purge the air from the gas lines
  • Inspect the heat exchanger wiring
  • Check the venting to ensure it meets specifications and that there are no blockages
  • Verify that the fans are operating at the proper speed
  • Contact a licensed technician

Error Code EC

Ionization Failure

An Error Code EC is an indication that there’s an ionization failure during the operation of the unit.

  • Check the gas type and gas pressure
  • Check the heat exchanger’s wiring. The 3 wire connections on the lower front of the heat exchanger should be securely in place
  • Inspect the venting for blockages and that it meets specifications
  • Ensure that the minimum power fan speed is correct

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Error Code EO

Internal Hardware/Software Failure

An Error Code EO means there’s either an internal hardware or software issue. However, this error code can also occur if multiple buttons are pressed.

  • Reset the error code by pressing the reset button. If the error code does not trigger again, you’re probably fine
  • However, if the error code reoccurs, power the unit off and check the ground connections and the two wires on the control board, in addition to the cheater chassis ground connection
  • The unit should be connected to a 120v AC power supply, not a generator. In order to prevent voltage fluctuations and interferences, it needs to be on a dedicated circuit
  • Contact a licensed technician to further troubleshoot the problem

Error Code F7

Ionization Rod 

An Error Code F7 indicates an ionization error occurred when the unit was in standby.

  • Inspect the flame ionization rod for loose connections. The thinner wire coming from the control unit must be securely connected to the set of electrodes on the lower front of the heat exchanger
  • Inspect the flame rod and control unit to ensure they’re in good working condition
  • Contact a licensed technician

Error Code FA

Gas Valve Circuit

A Bosch Error Code FA indicates that the gas valve circuit isn’t closing, which is therefore causing a gas leak.

  • Unplug the power supply cord
  • Inspect the wire connections on the gas valve 
  • Inspect the two connectors on the control board
  • Check your users manual for details on how to check the voltage at the gas valve wire plug connection. You may need to contact Bosch Water Heating for additional troubleshooting
  • The gas valve may be defective. Contact a licensed technician

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BY M. Kogan

Hello, I am Marcio. I am an architect and designer, alma mater is Mackenzie. Retired in theory, but an architect never retires completely. Along with architectural projects, I am a filmmaker and have completed some short documentaries. Filmmaking and design are my passions. In HomeQN I write about home decoration and foundations. The goal is to teach homeowners to DYI as much as possible, and when this is not possible, enable them through knowledge, to evaluate service quotations and choose the best service technicians.

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