Burner shuts off before set temp is reached. Blower continues to run and igniter comes on to light gas but before gas turns on blower stops. Gas then turns on and lights but when blower comes back on, flames dances and will repeatedly shoot flame out of heat exchanger. If the gas valve and burners do not shut off and continue to burn until set temp is reached, burner flame is steady and no flame shoots out of heat exchanger. Is there an error code posted and where can I find it?