Auxiliary fan

Hey guys. I have a 2006 GMC Sierra with 90,000 miles and a 5.3 engine. Never any issues but I’ve noticed that when I start the truck up the electric auxiliary fan comes on immediately. This is when the truck is cold and the fan should not be needed. Never had any issues with it overheating or anything. Is this just a sensor gone out or something or is it normal? If it is a sensor which one is it and where would it be located?

Is it the AC fan? Defrost brings AC on automatically.

No, this is NOT the AC fan. It’s the one out in front of the engine at the radiator.

The temp sensor may be bad.

+1 to knfenimore’s post.

The cooling fans are controlled by three relays depending on the signal the PCM receives from the coolant temp sensor to the PCM.

Open the hood and locate the fan relay box. Switch any two relays and if the other fan runs when the engine is cold replace that relay. Because the contacts in that relay are probably stuck closed.


I think what kfenimore is asking is not about the cabin fan; it’s whether or not the radiator cooling fan is operating when the climate mode control is in the A/C, Bi-Level, or Defrost positions.

In case there is some misunderstanding turn the climate control off and note if the fan problem goes away.

Does the fan turn off after awhile? I am going out on a limb here but if my memory serves me correctly the ecm would command the fan on after startup in certain conditions. There were also relay issues but the fan would not shut off.

Thanks guys for all the leads, I will look into the relays. This may be a normal thing as the fan does turn off after a while.

It may be a normal GM thing across the board. My Trailblazer engages the electro-viscous engine fan at cold start up too. When they start to go bad, you can really notice it. I always thought it was a really strange implementation as the fan goes to highest speed immediately upon starting in sub-zero weather and then gradually backs off. The only reasoning I could come up with is it is a fail safe mode…