HI,HOPE YOU CAN HELP ME OUT.I HAVE A GOODMAN MANUFACTURING FORCED AIR FURNACE THAT I BELEIVE IS GIVING ME A WARNING SIGN OF A FUTURE FAILURE.WHAT HAS HAPPENED IS IN THIS ORDER,1st ONE MORNING THE THERMOSTAT SHOWED THAT THE FURNACE WAS ON AND IT WAS'NT.TURNED IT OFF AND THEN BACK ON AT THE THERMOSTAT AND THE FURNACE TURNED ON LIKE NOTHING WAS WRONG.2nd ANOTHER MORNING FOUND THE FURNACE ON BUT THIS TIME ALL IT WAS DOING WAS BLOWING COLD AIR(WENT TO LOOK AT FURNACE AND FOUND BLOWERS ON BUT NOT THE BURNER).ONCE AGAIN TURNED IT OFF AT THE THERMOSTAT AND BACK ON AGAIN AND IT WORKED AGAIN.3rd AND MOST IMPORTANT I FEEL IS THAT WHILE TAKING A GOOD LOOK AT HOW THE FURNACE WAS OPERATING AT START UP I NOTICED THIS-EXHAUST BLOWER TURNS ON,IGNITOR ACTIVATES,BURNERS LITE,3-5 SECONDS LATER THE BURNER SHUTS OFF(LIKE THE GAS IS SHUT OFF SUDDENLY),WAIT 20-30 SECONDS AND THE IGNITOR RE-ACTIVATES AND THE BURNER LITES.ONCE AGAIN 3-5 SECONDS LATER THE BURNER SHUTS OFF,WAITED AND WATCHED 20-30 SECONDS AND ONCE AGAIN THE IGNITOR RE-ACTIVATED AND LIT THE BURNER AND FINALLY ON THE THIRD TIME THE BURNER STAYED LIT.NOW ,THE THERMOSTAT WAS REPLACED BEFORE THE HEATING SEASON(NOTHING WRONG WITH OLD ONE JUST WANTED ONE MORE UP TO DATE)AND I RECENTLY EXCHANGED IT FOR A NEW ONE THINKING THAT MIGHT BE THE PROBLEM.WELL THE PROBLEM STILL EXISTS,VACUUMED OUT FURNACE AND REPLACED FILTER,CHECKED THE LIGHT FOR ANY FLASHING CODES AND ALL THAT IT WAS DOING IS A STEADY PULSE THAT INDICATES NORMAL OPERATION.THIS PROBLEM IS NOT VERY CONSISTENT THOUGH,SOMETIMES IT OPERATES JUST FINE.HOPING NOT TO BE IN THE COLD IN THE FUTURE,THANKS CHRIS...

Dirty flame sensor probably