Overheats if left idling but not if driving...cooling fans aren't coming on...what couls be wrong?

The cooling fans on your car are comanded on by the engine control module which reads engine temperature from the temperature sensor . when comanded on the engine control module sends a signal to the cooling fan relays . which in turn runs the current to the cooling fans . So you might want to check the cooling fan relays and make sure the fues for the cooling fans are good and the relays .if the cooling fan fuses and relays are good it could be a problem with the fan motors or the engine control module not reading the right temperature in that case it could mean you have a problem with the temperature sensor

Your fans could be seized which is not uncommon for gm vehicles.if you start the car and turn the a/c on the fans should work when the compressor kicks on if they don`t then they are probably no good you can tap on the motors with something when they should be running and if they start to work then you need new fan motors, you can power up the fans directly by using jumper wires to each fan motor one power and one ground wire again if they don`t work and you have made good connections then the motors are no good,other things could be a bad fuse,bad relay bad connections.the reason its not overheating when driving is there is enough air flow over the rad to keep the motor cool