Testing Coolant Switch Or Sensor
If both fans run when you hot-wire them, you know the problem is somewhere between the fuse and the cooling fan motors. On most systems, the there is a coolant temperature switch that turns the fans on and off. Locate the coolant temperature switch and unplug it. Be sure you have the right one. Some vehicles have as many as three: one for a dashboard warning light, one for an overhead console and one for the PCM

Now, with the engine running and the coolant temperature switch unplugged the fan(s) should come on. The PCM will detect a failed switch, store a Diagnostic Trouble Code (DTC) and turn on the fan(s). If it doesn't, and you have a domestic vehicle, the coolant temperature switch is a normally open type that stays open until the coolant reaches it's set temperature. When it reaches that temperature, the switch closes and turns the fan(s) on. To check this, unplug the single wire connector and, using a jumper wire, ground it. The fan(s) should come on. Most Japanese cars have a normally closed switch. These switches open when the set temperature is reached thus turning the fan(s) on. If your car has this type of switch, simply unplugging with the ignition on, should turn the fan(s) on.

Most late model cars are equipped with a Coolant Temperature Sensor (CTS). This is not a switch. It is a temperature-sensitive resistor (thermistor) that sends a signal to the PCM. The PCM takes this signal and decides at what speed the fans will run and when. The PCM also has some control over the A/C and uses inputs from the A/C to determine cooling fan(s) operation. Testing a CTS is not as easy as unplugging it or grounding it and the cooling fan(s) will not respond.