# furnace, auto fan not working



## Ken A. (Oct 2, 2011)

the burner comes on but the auto fan won't come on
we can turn on the fan manual and that works OK
the fan works fine on air conditioner auto


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## JJboy (Oct 12, 2010)

probably your flame sensor is dirty or has ground problem.


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## sammy37 (Dec 9, 2008)

JJboy said:


> probably your flame sensor is dirty or has ground problem.


He says the burner is running. Probably has a bad fan/limit switch.


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## JJboy (Oct 12, 2010)

sammy37 said:


> He says the burner is running. Probably has a bad fan/limit switch.



I know, but he also says the blower motor is working in cooling mode and auto.


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## Tator1076 (Dec 22, 2009)

Ken A. said:


> the burner comes on but the auto fan won't come on
> we can turn on the fan manual and that works OK
> the fan works fine on air conditioner auto


It can be a fan relay switch or limit. If it has control board then it a relay on the board. You be able take board of bracket and see if burn up on backside .


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## HvacWiz (Nov 24, 2006)

*Depending on what type of furnace it is, Its either the fan limit switch or the circuit board.*


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## Doc Holliday (Mar 12, 2011)

I don't even understand the problem. :huh:


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## sammy37 (Dec 9, 2008)

JJboy said:


> I know, but he also says the blower motor is working in cooling mode and auto.


He says if he puts the fan from auto to on that it works. It also works in cooling mode but it wont turn on in heat, that tells me that the fan/limit switch which is seperate from the fan relay, which controls the cooling and fan on, is bad.

I recently worked an old Carrier gas pack with the same issue and instead of fixing the bad part, the last tech on the job set the back of the thermostat from gas to electric, that way in heating mode it was relying on the fan relay. This is bad though because the heat exchanger doesnt get to cool down after the call for heating is over, just instant stop.

I had to replace the fan/limit switch in that and all was well.

And like the others said, if the unit has a board, that could be bad also.


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## JJboy (Oct 12, 2010)

sammy37 said:


> He says if he puts the fan from auto to on that it works. It also works in cooling mode but it wont turn on in heat, that tells me that the fan/limit switch which is seperate from the fan relay, which controls the cooling and fan on, is bad.
> 
> I recently worked an old Carrier gas pack with the same issue and instead of fixing the bad part, the last tech on the job set the back of the thermostat from gas to electric, that way in heating mode it was relying on the fan relay. This is bad though because the heat exchanger doesnt get to cool down after the call for heating is over, just instant stop.
> 
> ...


I agree, but I want to see if OP will tell us if burner stay on for a couple minutes or turn on/off quickly


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