I have a 2000 Saturn Wagon and the fan does not kick on to cool the engine, but it turns on when I run the AC.

The fuses are fine, the sensors are in good shape. All the basics surrounding this problem have been looked at. And we know the fan works because it turns on when we put on the AC.

Would it be an error of some sort in the computer that runs the cooling fan? And where is the computer located that runs the cooling fan?

Any and all advice is welcome. We're stumped.

So fan relay is good, fan is ok.

Try replacing the ECTS (coolant temp sensor) and connector (purchase from GM dealer).

PCM infers coolant temp from the resistance of this sensor. When it fails, it usually does so in such a way as to tell the PCM that the coolant is much colder than it actually is......if the PCM doesn't think the coolant is hot, no fan.......

+1 for the DERFster -- he's got the answer