The thermostat, housed in the coolant system, controls the temperature inside the engine. When the engine is cold, the thermostat is closed to allow coolant to flow just around the engine block, helping it warm up faster. When the engine reaches operating temperature, the thermostat opens to allow the coolant to enter the radiator so it can be cooled. A thermostat that is stuck open, allowing the coolant to pass into the radiator prematurely will cause the engine to take longer to warm up, and the fan might not turn on when it is supposed to.
Coolant temperature sensor (switch) failure
A coolant sensor is located in the cylinder head and measures the coolant temperature as the engine is running. If the sensor is faulty, the fan will not be triggered even if the coolant is boiling hot, as it will incorrectly tell the control module that the engine is cold. Some sensors simply fail, while others give false readings. A cooling fan sensor in a 1992 Volkswagen Cabriolet is located on the bottom of the radiator, usually screwed into one of the bottom water pipe connections or the water pump housing.
Wiring fault
Electrical issues can also prevent a cooling fan from working properly. If a sensor cannot transmit a correct signal due to a faulty or loose wire, the control module will not know when to trigger the fan. Similarly, a faulty relay can block power getting to the fan, even when the control module has signaled it to turn on.
Fan fuse blown
If the fan fuse has blown, the fan will not be able to receive any power regardless of any signals from the control module. Locate the fuse box, usually in the engine bay, and inspect the fan fuse.
Fan relay failure
In vehicles equipped with dual electric cooling fans, a fan relay is utilized to control the second fan. Should this relay be defective, or suffer a loss of power, the fan will not run as intended. The fan relay is usually found with other relays under the hood.
ECM failure
The control module is a final item that might be faulty. If all of the other reasons have been eliminated, and the wiring is intact and free from corrosion, then the control module may be responsible for the fan not working.
