A car's electric cooling fans are responsible for ensuring air flow over the radiator at low speed, and failure will almost always cause overheating while in traffic. Fortunately, these systems are fairly easy to diagnose in the event of failure.
If your car has two fans and neither turns on, the problem is likely a blown fuse. The fan fuse can be located in either the in-cab fuse box or the under-hood box.
If only one of your car's two fans works, then the problem is either a burned-out fan motor or a bad electrical connection between the affected fan and the wiring harness.
If your fans do not engage and the dashboard temp gauge reads normal, then the thermostat sensor is bad. A bad sensor will tell the car that everything is normal while the engine overheats.
If your car's fan does not turn freely by hand when the engine is off, then the fan motor is seized and needs to be replaced.
Fans may fail to engage if the car's system voltage is too low, as might be the case with a failing alternator.
