I need some help. My girlfriend has a 2002 Honda Civic that keeps overheating. The fan doesn't turn on. We tested both fans and they work properly. We changed the thermostat, the cooling fan switch and the fuse, the fan still doesn't come on when it reaches temperature.
Test the coolant temperature sensor. Most vehicles have 2 temperature coolant sensors, one for the instrument cluster and one for the ECU. A repair manual for your vehicle (Haynes or Chilton) will tell you where it is and how to test it.
Pull the cooling fan relay. Check for power on terminals 2 and 3 and ground on terminal 1 and on terminal 3 when the engine is hot.
Terminals 1 and 3 are the output to the cooling fan and the switch ground.
Terminals 2 and 4 are fused power.
If you're missing any of these when the engine is hot work backwards from the relay. Inspect the wiring. Check continuity. Recheck your fuses. Check the ground side of the cooling fan switch.