Troubleshooting Furnace Problems

Pilot won’t stay lit. Hold down the button, light, hold for a minute, let up. If the pilot goes out, it’s the thermocouple. This is a $7 part.

Pilot stays on but the furnace doesn’t come on, step 1. Do you hear a click when you raise the thermostat setting? If no, the transformer failed. This is a $15 part.

Pilot stays on but the furnace doesn’t come on, step 2. Push the reset button on the limit switch and try raising the thermostat setting again. The limit switches on older homes don’t have a reset button.

Pilot stays on but the furnace doesn’t come on, step 3. Replace the limit switche(s). You MUST know the brand and model number of the furnace to get the proper replacements. It will be helpful if you have the part number of the switches.

Pilot stays lit, fire comes on, but the fan doesn’t come on. Possible the fan switch. This is an $8 part located on the lower side of the furnace by the fire box. There is no way to test it, you just have to replace it and see if that fixes the problem.

New fan switch doesn’t fix the problem. That leaves the motor.

Still doesn’t work? Assuming you have fuel, this would indicate a problem with the gas valve. It’s time to call in a professional; gas valve work is to dangerous.

More about motors. Motors sometimes fail slowly. As they reach the end of their life, various parts start to fail so the motor has to work harder to turn the fan. This creates more heat which eventually causes the motors thermal overload protector to shut it off. After 20-30 minutes the motor cools enough to run again. It may take a month or two for it to fail completely. If that is long enough to get you out of the heating season perhaps you can postpone the expense of a new motor until the next fall. The risk you take is that the motor will fail completely some cold night and you will be looking at an emergency service call.

{ 6 comments… read them below }
Ms Manning
During the last few months of winter my furnance just quit working one night, woke up to the house to be around 50 degree. Had a service man come out and he replaced a receptacle (I believe) and a new motor. Now it doesnt blow enough air out to heat the house. I made it through the rest of winter fine with some space heaters. Now I tried the air conditioner and it runs alllll the time and hardly any air will blow out. What is my problem? The service man at the time said he doesnt know what else can be the problem.
Paul
I think you need a new service guy. There are to many possibilities for me to say much more than that.

MaryAnn
I have a coleman gas furnace and the green light tells me that the air switch is defective. However, I was told that the booster switch was bad. I have gone through three booster switch already in a matter of a few short months, the last one just days. The last one wants to start but runs really slow if at all. What is causing the green light to say it’s an air switch problem but the booster switch keeps going out?
Paul
That’s a good question buy beyond my skill/knowledge level. It sounds like it’s time for a visit from an experienced mobile home HVAC guy.

tim Pedro
My problem is my heater turns on, but the fan does not blow heat out. It actually shuts off. The fan doesn’t turn on at all. The a/c works fine. Is there some kind of switch like a thermostat to get the fan going? I have a heat element, not a pilot light.
I believe that it is some kind of switch not working, but have no idea which one is problem. Please help me out with this since winter is not so far away. Sincerely, Timothy Pedro.
I have a Colman Evcon. I’ll be happy to give you anymore info you may want.
Paul
Its called a limit switch. It is supposed to turn the fan on when it senses heat and keep the fan running until the combustion chamber cools down.
MHParts.com should be able to send you the right one. They will need to know the furnace details including model number.

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