Troubleshooting Tankless Water Heater Error Codes: Bosch

Bosch is a well-known brand of tankless water heaters that are commonly used by many homeowners.

If there is a fault with your Bosch tankless water heater, a certain error code will be displayed on the unit’s display. This error code gives owners a window into what is going on inside the unit.

Not all error codes are necessarily bad, but every error code should be thoroughly looked into. 

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Assessing the situation and the reason behind the error code ensures the tankless water heater’s safe operation.

Water heater beeping? See our article Why is My Tankless Water Heater Beeping?

This article goes over the common Bosch tankless water heaters error codes and what you need to know about them.

Error Code A2: Problem With the Flue Gas Limiter

Bosch error code A2 indicates a fault in the flue gas limiter. To fix Bosch error code A2 check the continuity of the flue gas limiter and fix any problems. Ensure there is no flue gas leakage around the heat exchanger’s seals. Ensure the connections of the gas limiter and all wiring harnesses are secure.

To check the condition of the wiring, unplug the unit from the power supply. Ensure there are no loose or damaged wiring harness connections on the control board.

Error code A2 also means the unit may be overheating, as it usually indicates that the temperature inside the cabinet is above 230°F.

Error Code A3: Exhaust Temperature Sensor Is Disconnected

Bosch error code A3 indicates that the exhaust temperature sensor has been disconnected. If your Bosch tankless water heater displays an error code A3, ensure the exhaust temperature sensor is installed correctly and the connection is tight and secure. If necessary, check and replace the exhaust temperature sensor.

Additional information on how to measure your particular unit’s sensor can be found in your unit’s troubleshooting manual.

Error Code A4: Backflow Temperature Sensor Fault

Bosch error code A4 means there is a fault in the backflow temperature sensor. Bosch error code A4 is usually caused by a disconnected sensor or a short circuit. Ensure the backflow temperature sensor is installed correctly, measure its readings, and replace it if necessary.

You can find more information on troubleshooting your unit’s sensor in the unit’s installation manual.

Error Code A7: Outlet Temperature Sensor Fault

Bosch error code A7 indicates a problem with the outlet temperature sensor. Ensure the wire connections are in good working condition, free of corrosion, and installed tightly. Bosch error code A7 can occur if the water temperature drops below 36°F goes above 210°F, or the unit needs to be descaled.

As part of this error code’s troubleshooting process, you may need to measure the sensor’s resistance readings, too. A mode detailed, model-specific information can be found in your unit’s manual. If the sensor appears to be faulty, replace it.

Keep in mind that if the water temperature drops below 36°F, it will freeze. You need to take the necessary precautions to prevent that. The manufacturer’s warranty does not cover damage done by freezing water.

If the temperature of the unit goes above 210°F, error code A7 will show up. Usually, this overheating can be caused by scale build-up, meaning the unit should be descaled to prevent further overheating. Depending on the water quality, you may need to perform frequent descaling and treat the water to prevent scale build-ups.

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Error Code A9: Outlet Temperature Sensor Fault

Bosch error code A9 indicates that the outlet temperature sensor is not detecting the expected output temperature. Ensure the outlet temperature sensor is attached correctly to the hot water pipe. Check the gas pressure and the supply voltage the unit receives and that it is properly grounded.

When Bosch error code A9 appears on the unit, it usually flashes. In addition to that, it is, in fact, considered a status message rather than a typical error code.

Error code A9 often appears if the sensor is not attached correctly to a vertical section of the hot water pipe or low gas pressure or low voltage. However, if everything seems to be in check, then you may be dealing with a faulty control unit, which should be replaced.

If necessary, contact a licensed technician to assess the condition of the unit.

Error Code C6: Slow-Spinning Fan

Bosch error code C6 means the rotation of the primary fan is too slow. Bosch error code C6 can be caused by poor wiring, low voltage, low gas pressure, and incorrectly set or blocked venting. If everything works as intended, then this error code can be caused by a defective fan or control unit.

After resetting the error code and disconnecting the unit from the power supply, inspect and ensure the fan is wired correctly, there are no loose connections, and the unit receives enough voltage.

Thoroughly check the venting. Often long vents, a high number of elbows across the venting system, or blocked venting can cause error code C6. Cross-contamination between the exhaust and the air intake vents can also cause error code C6 to appear on the unit.

If everything is within specifications, contact a licensed technician.

Error Code C7: Fan Rotational Speed Sensor

Bosch error code C7 means the primary fan rotational speed sensor does not detect any movement. To fix Bosch error code C7, reset the error code, unplug the unit from the power supply, check the primary fan’s wiring and the unit’s supply voltage and grounding. Replace the sensor, fan, or control unit if needed.

Contact a trained technician if necessary.

Error Code CA: Excessive Water Flow

Bosch error code CA indicates that the water flow is over 10 GPM. If your Bosch displays error code CA, unplug the unit from the power supply and inspect the water valve’s wiring and the PCB. Ensure the water pressure flow is below 10 GPM, and the water pressure is below 150 PSI.

Error Code E1: Extremely High Temperature

Bosch error code E1 means that the outlet temperature sensor has detected high amounts of heat, and the temperature detected is over 203°F. Ensure the sensor is working properly, attached correctly to the hot water pipe, wired correctly, and the unit does not have scale build-ups.

More information about your unit’s sensor’s specifications can be found in the unit’s troubleshooting manual.

Error Code E2: Inlet Water Temperature Sensor Fault

Bosch error code E2 means there is a problem with the cold water temperature sensor. Bosch error code E2 can show up if the water temperature goes below 36°F. The cold water temperature sensor or the water valve may need to be replaced.

If, after resetting the error code, the error code persists or keeps showing up again, you should troubleshoot the unit to determine the root cause of the issue.

The sensor will normally trip if the inlet water temperature goes below 36°F, but it can also trip if recirculating water above 140°F is being fed to the unit.

The cold water temperature sensor’s resistance should be measured, and depending on the readings, the sensor or the water valve may need to be replaced. Keep in mind that the sensor’s location and recommended specs can vary between units. Detailed information about your unit’s sensor’s specs can be found in the unit’s manual.

Error Code E3: Problem With the Exhaust Temperature Sensor

Bosch error code E3 is found on condensing units and indicates a fault with the exhaust temperature sensor. The unit will automatically shut down and lock if the temperature readings go over 194°F. Ensure the heat exchanger and condensing heat exchanger is clean and working properly. Lower the maximum power or water temperature setting.

Error Code E4: Problem With the Backflow Temperature Sensor

Bosch error code E4 means that the backflow temperature sensor is over 309°F, at which point the unit will shut down and lock. If your Bosch displays error code E4, check all vent pipes and ensure they are within specifications and not blocked. Ensure the secondary fan wiring is working properly.

More information on the correct vent length and measurements can be found in the unit’s manual. 

If everything seems to be in good working condition, you may need to call for a service.

Error Code E9: Overheat Sensor ECO Has an Open Circuit

Bosch error code E9 indicates that the overheat sensor (ECO) has an open circuit. If your Bosch unit displays error code E9, unplug it from the power outlet. Ensure the wiring of the overheat sensor is in good condition, tight, and clean. Ensure the venting is within specification and not blocked.

Descale the unit if necessary.

To troubleshoot the overheat sensor when there is an error code E9, unplug the unit from the power outlet and open a hot water tap. Since the unit is not powered, only cold water should flow. After a few minutes, close the hot water tap and disconnect the overheat sensor’s lead wires.

Use a multimeter to find out if there is continuity and if the sensor is open, replace it with a new one.

Error Code EA: No Flame Ionization Detected With the Water Flow

Bosch error code EA means that no flame ionization has been detected with the water flow. If your Bosch displays an error code EA, ensure its wiring, gas supply, and venting is in good working condition and set up correctly. If everything seems in good condition and the error code persists, contact a licensed technician.

Troubleshooting Bosch error code EA involves several different steps.

  • Ensure gas is running to the unit.
  • Check the color of the flame after a hot water tap has been opened. You should see a stable blue flame. According to specifications, if the flame is unstable or yellowish, ensure the gas pressure, gas type, and CO2 readings.
  • Remove any air that may be within the system. This can be achieved by resetting the error code and opening and closing a hot water tap multiple times. However, you may need to contact a licensed technician to purge the air from the gas lines.
  • Check the wiring of the heat exchanger.
  • Ensure the venting is according to specifications.
  • Ensure the fans are running and their speed has been properly set up.

Error Code EC: Ionization Failure During Operation

Bosch error code EC means there is no ionization during operation. If your Bosch displays an error code EC, ensure it receives the right type of gas and enough gas pressure. Check the heat exchanger’s wiring, venting, and adjust the CO2 and minimum power fan speeds.

Error Code EO: Internal Hardware/Software Failure

Bosch error code EO means there is an internal hardware or software problem. If your unit displays an error code EO, power it off, and ensure that all wiring is set up correctly. Bosch error code EO can happen by pressing multiple buttons, too, in which case the error should be reset.

Always reset the error code using the reset button.

Usually, a single EO error code may not be a cause for concern. However, it is still recommended to inspect the unit’s condition, especially if the error code keeps showing up.

If the error code persists, ensure the unit is connected to a 120V AC supply and not a generator. Keep the unit on a dedicated circuit to prevent interferences and voltage fluctuations. Ensure the unit is properly grounded and wired.

If everything looks good, but the error code keeps showing up, contact a licensed technician to assess the condition of the unit.

Error Code F7: Ionization Rod Problem

Bosch error code F7 means there is an ionization error at standby. If your Bosch displays an error code F7, ensure the flame rod and control unit is in good working condition. Inspect for any loose wiring and connections at the flame ionization rod and the heat exchanger’s electrodes.

Error Code FA: Gas Valve Circuit

Bosch error code FA means the gas valve circuit is not closing, causing gas leakage. Suppose your unit displays error code FA, power it off, and inspect the wiring and connectors at the gas valve, control board, flame ionization rod, and the electrodes at the heat exchanger. Replace the gas valve if needed.

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Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.

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