Furnace working upon arrival. Customer said red light would flash 4 times when it wouldn’t work. Yellow light would flash sporadically when it was working. 4 red flash indicates ignition failure. Yellow light flash indicates poor flame sense. Pulled flame sensor, tested poor. Cleaned and retested, tested fine. watched for a heating cycle and saw no more issues or error codes. Informed customer of rust on burners and burner box. Performed combustion analysis and passed.