* Fan Motor Failure: The most straightforward reason is a failed passenger-side fan motor. These motors can wear out, especially with age. The windings might be burned out, or the motor's internal components could be damaged.
* Relay Failure: While both fans often share a common circuit *initially*, there might be separate relays controlling each fan (or a relay controlling one and the PCM controlling the other – this is more common in later models but possible even then). A failed relay for the passenger-side fan would prevent it from working.
* Wiring Issues: A broken, corroded, or loose wire in the passenger-side fan circuit is another common problem. Check the wiring harness connecting to the fan motor, the connector itself, and the wires leading to the relay or the PCM (Powertrain Control Module). Look for chafing, breakage, or corrosion.
* Fan Control Circuit: The issue could lie within the vehicle's cooling system control circuit. This could involve the coolant temperature sensor, the PCM (if it's involved in fan control), or even a faulty ground connection within the system. A faulty temperature sensor might not send the signal to activate the passenger side fan.
* Fan Switch/Module: Some vehicles use a dedicated switch or control module for the fans, which might be faulty.
Troubleshooting Steps:
1. Check the Fuses: While less likely given your description, it's always best to rule out blown fuses first.
2. Test the Passenger-Side Fan Motor Directly: This is the most crucial step. Disconnect the connector to the passenger-side fan motor and use a multimeter or a jumper wire and battery to test if the motor works. Be cautious when working with car electrical systems. Incorrectly connecting a battery can cause damage or injury. If the motor works when tested directly, the problem is elsewhere in the circuit.
3. Check the Relays: Locate the relays (usually under the hood in a fuse box). Swap the passenger-side relay with a known good one (from a similar circuit, if possible). If the problem moves to the other side, the relay is faulty.
4. Inspect Wiring: Carefully examine the wiring harness leading to the passenger-side fan motor for any damage, corrosion, or loose connections.
5. Check Coolant Temperature Sensor and PCM (Advanced): This requires more automotive electrical knowledge and tools. You may need a multimeter and a wiring diagram for your specific vehicle to check the sensor's readings and the PCM's outputs.
If you're not comfortable with electrical work, it's best to take your car to a qualified mechanic. They have the tools and experience to diagnose the problem accurately and safely.
