If your vehicle is showing radiator cooling fan never turns on, even when the engine is hot, here are the most likely causes and the parts you'll need to fix each one.
The relay acts as an electrical switch to send high-current power to the fan motor. If the relay fails or the circuit's fuse blows, the fan will receive no power regardless of engine temperature.
The electric motor that drives the cooling fan can wear out, seize, or burn up internally over time. Even if the computer commands the fan to turn on, a dead motor will not spin.
The engine computer relies on this sensor to monitor engine heat. If the sensor fails and reports a falsely low temperature, the computer will never command the cooling fan to turn on.
Many modern vehicles use a dedicated electronic module to control variable fan speeds. These modules are prone to overheating and failing, which completely disables the cooling fan.
If the vehicle recently had radiator, belt, or front-end engine work done, the mechanic may have forgotten to plug the electrical connector back into the fan motor assembly.
On older vehicles or specific cooling system designs, a dedicated temperature switch directly triggers the fan. If this switch fails in the open position, the fan will not activate.