A.O. Smith 110-310-510 – ERROR CODE 391 [510 (T-D2)], TWO FLASHES/39 [110 (T-KJR2), 310 (T-K4)] AIR?FUEL RATIO ROD FAILURE
1) Check that the wire connections are secure and free of burns or cuts.
2) Check that the clear plastic tube connecting the proportional gas valve to the combustion chamber is attached.
3) Clean the AFR (p. 25).