# Furnace fan cycles on and off



## Lrobin84 (11 mo ago)

Hello new member here. I have a trane xv90 furnace. The fan cycles on and off when on heat. I disconnected the air intake pipe and that didn't do anything. I disconnected the exhaust pipe and the furnace lit and worked fine. So I'm thinking there's a block somewhere right? I have jumped the pressure switch so it can run for now. I can see the smoke coming out of the exhaust pipe on the outside of the house. Do you guys think it's just a block somewhere in the exhaust or since the furnace works when I jumped the pressure switch, is the pressure switch bad? Thanks


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## GregHVAC (10 mo ago)

Hi Lrobin.. Are you still experiencing issues with your furnace cycling on and off? If so, you may want to check the flame sensor. If the flame sensor is dirty, it does not 'see' the flame and will shut itself off as a safety precaution. If you take a some steel wool and lightly clean the sensor, that could solve the issue. My company works in furnace repair and I've seen this issue. Hope it helps!


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## Steven123cool (Mar 2, 2021)

Lrobin84 said:


> Hello new member here. I have a trane xv90 furnace. The fan cycles on and off when on heat. I disconnected the air intake pipe and that didn't do anything. I disconnected the exhaust pipe and the furnace lit and worked fine. So I'm thinking there's a block somewhere right? I have jumped the pressure switch so it can run for now. I can see the smoke coming out of the exhaust pipe on the outside of the house. Do you guys think it's just a block somewhere in the exhaust or since the furnace works when I jumped the pressure switch, is the pressure switch bad? Thanks


Altitude matters and so does the pressure switch. Most forced air gas fired units have this safety to protect you from death, do not bypass or otherwise ignore what it is telling you. It means there is a blockage in the exhaust or intake side or the fire


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## Steven123cool (Mar 2, 2021)

Or partial blockage,flue too long or too many 90’s, birds nests? Installation instructions at very specific on pipe length and amount of 90’s


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