Radiator Fan Relays!
The radiator fan relay senses the coolant temperature then activates or deactivates the cooling fan based on the desired temperature for the engine to run at.
When the radiator fan relay fails your vehicle's cooling fan may not activate at all, or may not be consistent in how it performs.
Where the radiator fan relay is located will depend on your specific vehicle, but most commonly they are underneath the hood in a fuse box.
We stock and sell a wide variety of quality radiator fan relays from these manufacturers: