A.O. Smith 110-310-510 – ERROR CODE 121 [510 (T-D2)], THREE FLASHES/12 [110 (T-KJR2), 310 (T-K4)] FLAME LOSS

This error code appears if the flames disappear while the unit is still supposed to be in operation.
1) Verify that the hi?limit switch is not tripped. Turn the power off to the unit. Push the white button
in. The switch will reset when you hear and feel the button click. Turn the power back on and test
the heater. If the unit fires on, the cause of the hi?limit trip must be determined. Please contact the
Technical Service Department for further technical support.
2) Check the gas supply. Verify that sufficient gas is being supplied to the heater and the gas line is
cleared of debris.
a. It is possible there is a faulty pressure regulator at the gas meter.
b. For propane units, colder periods of the day result in a cold regulator and may cause this problem.
3) Verify that the gas supply pressure is within specifications when the heater is in standby and when the heater is running.
4) Check that the clear plastic tube connecting the proportional gas valve to the combustion chamber is attached.
5) If the flame comes on for 1?2 seconds but then goes out, clean the flame sensor (p. 25).
6) The overheat cutoff fuse may have been compromised. For procedures on how to check whether the OHCF is intact, refer to p. 24. If it has, contact the Technical Service Department for further assistance.
7) Check for blockages in venting, such as bird nests, animals, or trash.
8) PC Board fault or Gas Solenoid fault.
a. If one of these is suspected to be the problem, contact the Technical Service Department.