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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8681
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Furnace fan not working when thermostat is in Auto. Why?

Customer Question

My furnace blower won't run when thermostat is in the Auto position. When in the On position the blower works fine and the furnace works ie. heats. Is there a sensor on the furnace or something that could keep the furnace blower from kicking in or is this a possible thermostat problem?

Submitted: 5 years ago.
Category: HVAC
Expert:  Phil replied 5 years ago.

Phil :

Hello, there is the fan control relay operated from the primary solid state control card or a separate device?

Customer :

Is this fixable?

Phil :

Its easily fixable

Its not too expensive if its a separate relay

I need the model number from the unit name plate

It may be inside the area where the gas valve and controls are

That cover lifts up and then off

Customer :

I will get it

I believe it is one of these numbers 5800f44268 or g2003e-57-7
Phil :

Thanks, neither of those numbers check

For now we get by without them

Customer :

This is what I get from lenex site with 5800f44268

Phil :

First locate the fan motor. Two of the wires will go to a tin-can looking device.

Customer :
Part TypeFan Relay


Our Part Number
Use this number to order online. Receive additional discount and free shipping
Price $182.12
AvailableYes-In Stock
Additional infoFAN RELAY
Phil :

Two of the other three will go to the fan relay control

Lets be sure that is the right one

Find it on the wiring diagram glued to the inside of the furnace service panel

Then confirm that it looks like the one you just found on the web

We can go from there

Customer :

Looks like 2 wires going to a starter cap?

No picture on website

Phil :


Now where do the other three wires go

We can proceed from there

Customer :

There are 5 other wires. I will check

Phil :

In that case it may be an ECM type motor (variable speed)...let me know where each of the 5 wires go, and look the unit over again for the model number.