Central-air heater fan won't run, either in Auto or On mode. The heat comes on, but the fan doesn't. Even with heat switched off, the fan should run if I switch it On. The breaker is on, the wall switch is on. I eliminated the thermostat by wiring R and G together; still no fan. Next I'll look at the limit switch, but I'm wondering if a bad limit switch even comes into play if I turn the fan to On mode. So that's my question: is the limit switch used when the fan is in the On position? Doesn't seem like that would be necessary. It appears my fan motor may be bad (approx 15 years old). Thank you.

  • I have my central heat. On Auto click to heat .it not coming on. Oct 28, 2018 at 17:19
  • Have you checked to see if the blower motor is receiving power? Blower unit turns freely, possible internal thermal overload has not tripped and needs to be reset. Just a few things to add to your list.
    – Ed Beal
    Oct 28, 2018 at 18:31

2 Answers 2


If you tie the red and green together the fan should come on assuming the transformer wires are getting power (and it sounds like they are). That's the only thing the thermostat does when you put it in "On" mode. There shouldn't be and switches in the way.

The fan motor could have a start capacitor to kick it off, and those do go bad. That's be what I would check next. Also, the 24v current from the thermostat causes a relay to close which switches power to the fan. Those relays can also go bad. When the thermostat is turned to "On" mode, can you hear a click when you are right next to the blower (need a helper)? All things to check...


Could be:

  • Bad motor
  • Bad relay
  • Bad control board
  • Bad capacitor
  • Open door switch
  • Loose wiring
  • Blown fuse
  • Tripped motor thermal protection

WARNING: If you don't feel comfortable poking around in HVAC equipment, contact a local licensed HVAC technician.

There's no way to be sure, without poking around with a volt/multimeter.

Start by checking for voltage at the motor (you'll have to look up what speed the motor should be running at, as these units typically have multiple speeds). If you have voltage there, you'll have to test the motor windings and capacitor.

If you don't have voltage at the motor. Work your way back through the control board to the supply wiring, looking for where the voltage stops.

  • Thanks Tester101 and JPhi1618, very helpful suggestions. Unfortunately, after replacing the capacitor ($10) with still no luck, I decided to call a pro, and he says I need a new motor. :(
    – DJC
    Dec 22, 2015 at 19:02
  • Quick update: After the Technician purchased and installed a new blower motor, and the problem persisted, he realized he had not tested the relay, and turns out that was the problem. He did hear a 'click' when he initially checked the board (and I assume a 'click' indicates a good relay), but power was not getting past the relay. You guys are good!
    – DJC
    Dec 23, 2015 at 3:10

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