Hi.
The car model was not included in the question. In general if the radiator fan is not starting the first thing to check are the fuses. Depending on the car's model there is one or two fuses in line with the cooling fan circuit. If there are two fuses, then one is in the fuse plate inside the car, the other one is under the hood.
The next part to test is the thermostat. The thermostat housing is located on the refrigerant fluid lines, either near to the radiator, or near to the engine block. The thermostat can be tested using VOM tool. Read either Ohms or volts to find out if the thermostat is closing the fan circuit when the engine is hot.
Most fan circuits use also relay. Variable speed fans have often more than one relay. The fan relay(s) is often located under the hood. Relays can be tested reading volts at their contacts or swapping the relay with one of the same kind.
If fuses, relay and thermostat are OK, then the fan motor is often the next test. The motor can be tested energizing it directly. Variable speed fans have more than two wires powering the motor. To avoid risk of damaging the fan motor the repairer should refer to specific wiring diagrams before jumping fan cables. A different way of testing the motor is reading volts at fan motor while engine is hot. If there is volts at motor contacts but motor does not come on, then the fan motor is defective.
Regards.
Ginko
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