P1673 Fan output stage

The code P1673 indicates a problem with the fan output stage, but its specific meaning can vary depending on the vehicle manufacturer and model. The "output stage" refers to the electrical circuit or control module that activates the cooling fan. 

For most vehicles, P1673 relates to the engine or radiator cooling fan, but in others, it might refer to different systems, like the hybrid battery cooling fan in a Honda Civic or the immobilizer system in a Land Rover. 

Common causes of P1673

• Cooling fan or motor failure: A mechanically seized or electrically failed fan motor is a very common cause.
• Faulty fan control module or relay: The module that controls the fan speed or the relay that switches the fan on and off can fail.
• Open or short circuit: Damaged wiring in the fan circuit can result in an open circuit (no signal) or a short circuit (either to ground or power).
• Blown fuse: A protective fuse for the fan circuit can fail and needs to be replaced.
• Engine Control Module (ECM) failure: While less common, a faulty ECM can mistakenly report a problem with the fan's output stage.
• Vehicle-specific issues:
  • Honda Civic Hybrid: The code can indicate a fault with the hybrid battery sensor or its circuit.
  • Volkswagen: In some older models like the Beetle, it can point to an immobilizer or ECM communication issue.
  • Land Rover: On some models, it is linked to the immobilizer or security system. 

Symptoms of a P1673 fault

The symptoms you experience will depend on which system is affected.

• Check Engine Light: The most common symptom is an illuminated Malfunction Indicator Lamp (MIL) on the dashboard.
• Engine overheating: If the fault is with the engine or radiator cooling fan, the engine may overheat, especially in stop-and-go traffic or at idle.
• No fan operation: The cooling fan may not turn on when it should.
• Constant fan operation: In some cases, a short circuit can cause the fan to run continuously.
• Starting issues (for some vehicles): On certain Land Rover and Volkswagen models, an immobilizer-related P1673 can prevent the engine from starting.
• Hybrid system alerts (Honda Civic Hybrid): Hybrid system warnings may appear on the instrument cluster. 

How to diagnose and fix

1. Read the codes: Use an OBD-II scanner to confirm P1673 and check for any other related codes. Use a manufacturer-specific scanner if possible for a more precise diagnosis.
2. Inspect the fan: Examine the fan assembly for any visible damage or obstructions that could prevent it from spinning freely.
3. Check fuses and relays: Locate the fan's fuse and relay in the fuse box and test them. A blown fuse or faulty relay is a simple fix.
4. Test the fan motor: You can often test the fan by providing direct power to it from the battery to see if it operates. If it doesn't, the motor is likely bad.
5. Inspect the wiring: Look for any frayed, corroded, or disconnected wiring in the fan's circuit.
6. Use a multimeter: Test the voltage and resistance of the fan control module, motor, and wiring to pinpoint the source of the electrical fault.
7. Consult a professional: If you are not comfortable performing these checks or the problem is not a simple fix, take your vehicle to a mechanic. This is especially important for complex systems like immobilizers or hybrid batteries.