Error codes for Ultra Low Nox gas water heater


Inside the combustion chamber is a low nox burner.

The red and white wires from front of gas valve go down to the combustion chamber.
Generaly the wires connect to the 'heat sensor' or chamber sensor located on front of combustion chamber, then connect to the thermopile located inside combustion chamber.
The thermopile is set into the pilot flame. Heat from pilot flame causes an electric current that flows along the wires, back to the gas valve where the current powers the circuit board located inside gas valve.

The purpose of the sensor is to prevent overheated combustion chamber.
When heat inside combustion chamber exceeds 180-210° (or whatever the calibration temperature is), then the sensor trips off.

If sensor trips from overheating, then the electric current going to gas valve is off, and gas valve issues error code 10.
If thermopile fails, or pilot is not functioning correctly, or gas pressure is low, then gas valve issues error code 2.
If the sensor fails, then the gas valve issues error code 9, and the sensor needs to be replaced.

Error 4 is issued when temperature of water inside tank exceeds 180°F. Read about error code 4

Note that the error codes are not 100% ... some folks email that they fix and fix and fix their heaters and it still doesnt work even after replacing gas valve.