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Why is my car overheating at idle or in heavy traffic?

I have a 2002 Jeep Grand Cherokee. The check engine light is not on. However the car begins overheating when sitting at idle. The radiator fan does not run at any point. Could a bad thermostat cause the radiator fan to not run?

No, a bad temperature sensor or two is the likely culprit.
I suggest that you get this attended to very promptly.
Continued overheating is slowly but surely reducing the life of your engine.

A bad temperature sensor wouldn’t trigger the check engine light?

Maybe yes, maybe no.
In any event, the thermostat has nothing to do with the cooling fan failing to run, so you can dismiss that theory.

The radiator fan is not working. There is no way any manufacturer would have spent five cents on this assembly were it not absolutely necessary. Check your fuses first.

You probably have a bad fan relay or a bad fan control module. Does the fan(s) kick on when you put on the a/c.

The radiator fan does not run, the car overheats, yes oldtimer has it down unless of course the fan motor is shot. Question 1 is the fan motor run by a belt or an independant motor? Low coolant, and bad pressure are also possibilities.

Follow the wires from the fan (use the color codes on the wire). Find the relay and jump it out. If the fan runs, the relay is most likely bad. If it doesn’t it could be the fusable link or the fan is shot, or both (there’s typically a reason what the link goes bad.

The fan is wired hot, iirc. I believe that the relay provides the ground with a trigger from the PCM.

[quote]A bad temperature sensor wouldn’t trigger the check engine light?[/quoye]

Sure it can…the fan doesn’t kick on…the car over heats…the over heated can trigger the CEL.