Cooling Fans not working

Hi, i'm new to the forum. My girlfriend has a 1991 Lumina Euro 3.1. She got the car a few months ago and was bragging about how good the heat works. After doing a tune up on it I had noticed that her cooling fans don't work. I checked all the fuses in the glove box, under the hood on the passenger side next to the coolant resevoir, replaced the relays, and of course checked to make sure they were plugged in. The temperature gauge is reading right about in the middle. I'm not sure what else to check. Is there some type of sensor or sending unit? Any help is well appreciated.

Temperature sensor (located behind the rear head, near the egr pipe) - but verify that the fans work. If her a/c works, the second fan should engage with the a/c. One of the fans should also engage when you enter diagnostic mode (you know, the good ol' paper clip trick, at the aldl terminal)

remember too, the first fan shouldn't engage until the gauge reaches approximately half, then the second fan a few degrees after or if the a/c is on, or if the temperature is reaching near critical)

Mine don't come on until about 2/3 up the gauge. And it has to idle a long time before it gets there.

on the OBD1 cars, just jumper the diagnostic terminals to see if the main fan works.