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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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The fan on my furnace will not run in the auto position. It

Customer Question

The fan on my furnace will not run in the auto position. It will ignite but the fan does not come on so it ends up shutting the burners down. If we put the fan in the on position first it will work and then turn the heat on and stay lit. What could possibly be causing this. It seems like the fan works in every other position besides auto with the furnace.
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.


Please attach a well focused, close up photo of the wiring diagram glued inside the service panel,you can use the paper clip icon at the right end of the toolbar at the top of this dialog box,Include a picture of the thermostat wire connections to the printed circuit board in the same area.

If there is no paper clip icon, look for the '+attach files' link at left of your send button.

I will ask you to do a few tests using jumper wires and maybe a multi meter.

Customer: replied 1 year ago.
I have actually looked for a wiring diagram all over the unit and could not find one. It is an older unit ( around 1986) so I do not think it has a circuit board. I did not see one at least.
Expert:  Phil replied 1 year ago.

Hello again, did you remove the furnace blower door, so that you can see the actual blower inside? the wiring diagram is generally glued to the inside of that door.

The door lifts up one inch, then pulls out at the top in almost all cases, if there is a panel above it, you have to remove it first, the same way... sometimes there are two screws visible that you have to remove first (in less than 10% of furnaces)

Let me know what you find, we can go from there.

Customer: replied 1 year ago.
It looks like it's missing if it had it at one time.
Expert:  Phil replied 1 year ago.

Thanks, ***** ***** case send me 2 pictures of the furnace, with both service panes removed, taken from about 5 feet away that show the entire furnace... I will look at those and prescribe a few tests and probably a closer picture or two.