Error Codes and Troubleshooting Bosch Tankless Water Heaters

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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 be helpful for troubleshooting the problem:

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 is the limiter operating and wiring is properly linked to the control board and limiter. The element is located at the top-right corner of the unit.

A3 –Wire harness fault. Measure the resistance on wire harness; it must be between 115 and 120 ohms.

A4 – Check the backflow temperature sensor connection; it might be damaged or short circuit. It is located below the heat exchanger and ignition electrodes. Look for the diagnostic menu 1P and nC code.

A7 – Outlet temperature sensor fault and it reacts when the temperature is either below 36 F, when unit is protected from freezing temperatures, or above 210 F in a case of overheating due to lime-scale deposits. Check the sensor and wiring. You might have to flush the unit and remove the mineral build-up.

A9 – another error code associated with the outlet temperature sensor, but this time, sensor is not sensing the expected output temperature. This is not an error code, it is a flashing status message that tells you have to check the wiring connection, gas pressure might be too low, voltage doesn't meet required 120 VAC, the unit is not properly grounded or maybe the issue is with the control board.

C6 – Primary fan spins slow when the water heater is working. Check the wiring connection on the fan and control board. The voltage must be 120 VAC and element correctly grounded. Does the venting system meet the code and manufacturer requirements? Ensure the gas pressure is as required, because low pressure leads to fan rotation problems.

C7 – No signal from 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 properly grounded. Ensure the fan and control unit are not damaged.

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

E1 – Extremely high temperature, it is over 185 F detected by outlet temperature sensor. Ensure the sensor is working properly and the wire connections are not loose. Another reason for high temperature is the sediment build-up, so descaling is needed.

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E2 – Faulty inlet water temperature sensor. Check the sensor and wiring connections as this sensor is in charge of protecting the unit from freezing.

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.

E4 – Problem with the backflow temperature sensor. It reacts when the temperature is over 309 F. Check the vent pipes if meet the code and requirements, look for damages and obstructions.

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.

EA – 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, fan speed is low, wire connections are loose, air in the gas line, no gas.

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

E0 – The issue with the hardware or software. First try to reset the error code, there might be confusion inside the computer. Check are the wire connections properly linked to the computer board or the board is not damaged.

F7 – The problem is with the ionization rod at standby. Check connection to the electrodes, it should be securely connected. Other reason might be that the element or control board is damaged.

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

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