1. Thermostat:
* Stuck Closed: The most common culprit. A stuck-closed thermostat prevents coolant from circulating and reaching the proper temperature to trigger the fan. The engine might overheat, even if the fan isn't running. This needs to be replaced.
2. Cooling Fan Relay:
* Faulty Relay: The relay acts as a switch, powering the fan. A bad relay will prevent the fan from receiving power. You can swap it with another relay of the same amperage (check your owner's manual or a wiring diagram) to see if that fixes the problem. If the fan works with the swapped relay, you need a new one.
3. Cooling Fan Motor:
* Burned-out Motor: The fan motor itself could be faulty. This is less likely if the fan doesn't even run when you jump the wires (see below). Testing this requires more advanced tools or taking it to a mechanic.
4. Fan Switch (Temperature Sensor):
* Faulty Sensor: This sensor tells the system when the coolant is hot enough to turn on the fan. A bad sensor won't send the right signal. This can be tested with a multimeter to check the resistance at operating temperature.
5. Wiring and Connections:
* Loose or Damaged Wires: Check the wiring harness connecting the fan, relay, sensor, and ECU (Engine Control Unit) for any breaks, loose connections, corrosion, or damage. A visual inspection is key.
* Blown Fuse: Check the fuse box for a blown fuse related to the cooling fan circuit (consult your owner's manual).
6. ECU (Engine Control Unit):
* ECU Failure (Less Likely): Though less common, a faulty ECU could be preventing the fan from receiving the signal to turn on. This is a complex issue requiring professional diagnosis.
Testing the Fan Directly (Important):
Before replacing parts, try this:
1. Locate the fan's power wires. (Consult a wiring diagram for your specific year and model Civic, available online). Usually, there are two large wires going to the fan motor.
2. Directly apply power to the fan motor. Use jumper wires and a battery or a suitable power source (carefully!). This will bypass the relay and other components. Important Safety Note: Be cautious when working with automotive electrical systems. Make sure the power is disconnected from the battery before handling wires. Improper connection can cause damage or injury.
* If the fan runs: The problem is likely with the relay, the fan switch (temperature sensor), the wiring, or the ECU.
* If the fan doesn't run: The problem is likely with the fan motor itself.
Recommendation:
Start with the simplest and most likely culprits: the thermostat and the cooling fan relay. Checking the fuses and visually inspecting the wiring is also recommended. If you're not comfortable working with car electrical systems, take your Civic to a qualified mechanic for diagnosis and repair. Ignoring a non-functioning cooling fan can lead to engine damage from overheating.
