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I know there are similar questions out there but are there any new ideas?

When I turn on my heater, the gas fires up but the fan doesn't turn on. I know the fan is still working because it will turn on with the a/c on auto or the fan just by its self in "on" mode.

Thanks for any help

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    What make and model is your furnace? Can you post a wiring diagram for that matter? – ThreePhaseEel Nov 29 '17 at 5:15
  • Could be a problem with the furnace control board, or one of the coils in the blower motor. Do you have a multimeter, and are you willing to poke around in a live HVAC system? – Tester101 Nov 29 '17 at 12:17
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Bad Blower Motor Coil

Furnaces typically have multispeed blower motors, and use different speeds for different modes. If the coil for the speed the heat uses is dead, that would cause the problem you're seeing.

You can use an ohmmeter (or multimeter set to measure resistance) to check if the motor coils are good. You'll have to measure the resistance of each coil (while isolated from the rest of the system), and compare the results to the manufacturer's listed values.

Bad Control Board

There are a few things that could go wrong on the control board, that could lead to the issue you're seeing. The first thing to do, is to determine if power is ever getting sent to the motor at all.

THIS IS DANGEROUS
Since you'll be working on a live HVAC system, this testing can be dangerous. If you're not comfortable, please contact a local licensed HVAC technician.

Set your multimeter to measure AC voltage, and connect the common lead to the neutral terminal on the system. Hold the door lockout switch in, so that the system will try to fire. Once the burners ignite, listen closely to hear any relays closing.

Most furnaces use timers to delay the blower, to allow the heat exchanger to heat up. Once the preset delay has elapsed, a contact should close and send power to the HEAT terminal on the control board.

Put the other lead of your multimeter on the HEAT terminal, and see if you get a voltage reading. If you do, then the problem is with the motor or the wiring to the motor. Check all connections, to make sure they are clean and secure. If you don't read voltage, you'll have to either troubleshoot the control board, or simply replace it.

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