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I have a Coleman DGAT075BDD furnace that blowed the 3 amp fuse a few weeks ago so I replaced it and has worked fine until last night. I woke up cold and found the fuse blown again so I replaced it but the fan came on and would not go off and it was flashing a code 4. I started checking everything I thought it could be and foung that the upper limit switch has a reset and was triped. I reset it and repowered the unit and it has worked all day. My question is what caused this, is the board going bad or maybe the limit switch. Any help would be greatly appreciated.

Terry

Lets move this to the heating section.

Greg

Is there an air conditioner connected to this furnace? Is there a Blend Air system connected to this furnace?

Yes it has the blend air system and it does have A/C. I checked to be sure the blend air was working after I got the heat working again and it was working.

Disconnect all the Blend Air wiring on that unit. There has been a few instances where the Blend Air control board confuses the furnace's control board, sometimes it even causes it to fry out. Fortunately this time it was only the fuse. I am a Coleman warranty rep and did receive a service bulletin about this many years ago. I always pull the Blend Air systems whenever I run into them on any unit I service.

Can you tell me which wires I need to disconnect?

There will be a red, white, green and yellow wire coming from the Blend Air box. Those are tied into the thermostat wiring. Just remove those from the thermostat wiring.

I did disconnect the blend air system and have had no more problems so far so maybe that was the problem. Thanks DaleM and GOD BLESS.