Glow Plug Control Module Communication Failure
The P0684 code indicates a communication issue between the glow plug control module and the powertrain control module (PCM). This typically occurs when the glow plug module fails to respond or sends incorrect signals, often due to wiring faults, poor connections, or a defective module. This can affect cold start performance and trigger the check engine light.
1. Most Common Causes
- Faulty glow plug control module
- Open or shorted wiring in the communication circuit
- Corroded or loose connectors
- PCM software or hardware issue
2. Symptoms You Will Notice
- Check engine light illuminated
- Hard starting in cold weather
- Excessive smoke on startup
- Reduced engine performance
3. Step-by-Step Diagnosis
Step 1: Scan for codes and verify P0684 is present.
Step 2: Inspect wiring and connectors between glow plug module and PCM for damage or corrosion.
Step 3: Test communication circuit continuity and resistance with a multimeter.
Step 4: Check glow plug module power and ground supplies.
Step 5: If wiring is good, replace the glow plug control module.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
You can drive short distances, but cold starts may be difficult and the vehicle may fail emissions. It's best to repair promptly.
How much does it cost to fix?▼
Diagnosis typically costs $50-$150. Replacing the glow plug module can add $200-$500, depending on the vehicle.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.