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OK most likely a relay or control board in furnace. Do you have a multi meter so we can troubleshoot?Also I need model on furnace to give you specific guidance
Can you give me complete model from inside furnace in burner compartment so I can provide guidance with this specific unit.Model will be on a label just above burners on right or left inside panel
The 90 is just a series containing several models
Thanks for that information. I would really want to verify by checking to make sure we do not get voltage to motor on call for heat between terminals. H-Heat and CIR- N on board before diagnosing, but the fan is controlled by relays built into the board. So since it runs in on position (which is controlled by a different relay on board), but not in auto, I am comfortable in saying the relay is stuck open and not telling blower to come on in auto mode.
This require replacing the board. If it is something you want to do yourself and need help locating a board let me know.
If you have further questions, please let me know.
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To verify you can obtain a meter and check for 120v between HEAT-H and CIR-N for 120v 30 seconds after burners come on.If no 120v board is definitely the culprit.