Error Codes and Troubleshooting Bosch Tankless Water Heaters

Error codes are the indicators of the problems inside the water heater. When the error code appears on the Bosch heater's LCD display, the reset button sometimes leads to an error removal, but if it doesn't work, the instructions below might help troubleshoot the problem: 

Bosch tankless water heater

Error code A2 - Problem with the flue gas limiter

The temperature of the flue gases is too high and is above 230 F. It is important to check for any gas leakage close to the gas combustion chamber and heat exchanger. Check if the limiter is operating and wiring is correctly linked to the control board and limiter. The element is located at the top-right corner of the unit.

Error code A3 – Wire harness fault 

Measure the resistance on the wire harness; it must be between 115 and 120 ohms.

Error code A4 - Backflow temperature sensor

Check the backflow temperature sensor connection; it might be damaged, or it got short-circuited. It is located below the heat exchanger and ignition electrodes. Look for the diagnostic menu 1P and nC code.

Error code A7 – Outlet temperature sensor fault

The outlet temperature sensor reacts when the temperature is either below 36 F when the unit is protected from freezing temperatures, or above 210 F in a case of overheating due to the lime-scale deposits. Check the sensor and wiring. You might have to flush the unit to remove the mineral build-up.

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Error code A9 – Outlet temperature sensor fault

Another error code associated with the outlet temperature sensor, but this time, the sensor is not sensing the expected output temperature. This is not an error code; it is a flashing status message that tells you to check; the wiring connection, gas pressure as it might be too low, voltage as it doesn't meet the required 120 VAC, the unit that might not be properly grounded or the issue with the control board.

Error code C6 – Slow-spinning fan

The primary fan spins slowly when the water heater is working. Check the wiring connection on the fan and control board. The voltage must be 120 VAC and the element correctly grounded. Does the venting system meet the code and manufacturer requirements? Ensure that the gas pressure is as required because the low pressure leads to fan rotation problems.

Error code C7 – Fan rotational speed sensor

No signal from the fan's rotational speed sensor. The sensor might be broken. Check the wire connection on the control board and fan. The voltage needs to be 120 VAC and adequately grounded. Ensure that a fan and control unit are not damaged.

Error code CA – Excessive water flow

Make sure that the water pressure is less than 150 PSI and a flow rate below 10 GPM. Check the wire connections on the water valve and control panel.

Error code E1 – Extremely high temperature

 The temperature detected by the outlet temperature sensor is over 185 F. Ensure that the sensor is working correctly, and the wire connections are not loose. Another reason for the high temperature is the sediment build-up, so descaling is needed.

Error code E2 – Faulty inlet water temperature sensor

Check the sensor and wiring connections as this sensor is in charge of protecting the unit from freezing. 

Error code E3 – Problem with the exhaust temperature sensor

The unit will shut down if the temperature of the exhaust gases is above 194 F. Check and clean the heat exchangers if needed. Reduce the maximum power and water temperature.

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Error code E4 – Problem with the backflow temperature sensor

The backflow temperature sensor reacts when the temperature is over 309 F. Check the vent pipes if they meet the code and requirements, look for damages and obstructions.

Error code E9 – Overheat sensor ECO has an open circuit 

Check the wire connections on the sensor and control board. Ensure the venting system is per codes and instructions. Descale the unit if necessary.

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Error code EA – No sparks for the electrodes

The ionization electrodes are not making sparks when water is flowing. There are many reasons for this problem and here are some; low gas pressure, the fan speed is low, wire connections are loose, the air in the gas line, no gas.

Error code EC – Electrode failure

Ionization electrode failure during water heating. The reasons might be; obstructed venting system, low gas pressure, and CO2 readings, the fan speed is too low.

Error code E0 hardware/software issues

The issue with the hardware or software. First, try to reset the error code; there might be confusion inside the computer. Check if the wire connections are properly linked to the computer board or the board is not damaged.

Error code F7 – Ionization rod problem

The problem is with the ionization rod at standby. Check connections to the electrodes; it should be securely connected. Other reasons might include damaged elements or a control board.

Error code FA – Gas valve circuit

The problem is with the gas valve circuit. Check the wire connections on the gas valve and control board. Measure the voltage on the wires when a water heater is ON… it should be 24 VDC. If the measurement is as specified, the gas valve is malfunctioning.

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Bosch and minimum water flow 
Had a Bosch heater for 15 years. In the last 10 years, keep getting error code A2 - flue gas temperature too high. Sometimes I have to reset several times …

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