Glow Plug Control Module to PCM Communication Circuit Range/Performance
P0684 indicates a communication problem between the Glow Plug Control Module (GPCM) and the Powertrain Control Module (PCM). This code sets when the PCM detects a signal that is out of range or performing abnormally on the glow plug circuit, typically affecting diesel engines during cold start conditions.
What You Might Notice
- Check engine light illuminated
- Difficulty starting in cold weather
- Engine misfire or rough idle after cold start
- Reduced engine performance until warm-up
- Potential intermittent starting issues
Most Common Causes
- 1
Faulty Glow Plug Control Module
The GPCM may be failing or providing incorrect signal voltage to the PCM, causing communication errors. This is the most common cause of this code.
- 2
Damaged wiring or connectors
Corrosion, loose connections, or damaged wiring between the GPCM and PCM can interrupt proper communication signals.
- 3
Failed glow plugs
One or more defective glow plugs can cause the GPCM to report abnormal circuit conditions to the PCM.
- 4
PCM software issue or PCM failure
Outdated PCM software or a failing PCM itself may misinterpret valid glow plug circuit signals.
How to Diagnose It
- 1
Check Faulty Glow Plug Control Module — The GPCM may be failing or providing incorrect signal voltage to the PCM, causing communication errors. This is the most common cause of this code.
- 2
Check Damaged wiring or connectors — Corrosion, loose connections, or damaged wiring between the GPCM and PCM can interrupt proper communication signals.
- 3
Check Failed glow plugs — One or more defective glow plugs can cause the GPCM to report abnormal circuit conditions to the PCM.
- 4
Check PCM software issue or PCM failure — Outdated PCM software or a failing PCM itself may misinterpret valid glow plug circuit signals.
How to Fix It
- 1
Inspect and repair wiring and connectors
Check the wiring harness and connectors between the GPCM and PCM for corrosion, damage, or loose connections. Clean contacts and repair any damaged wiring.
- 2
Replace Glow Plug Control Module
If wiring and connectors are intact, test the GPCM with a multimeter or diagnostic scanner. Replace it if it's not sending proper voltage signals to the PCM.
- 3
Replace failed glow plugs
Test individual glow plugs for continuity and resistance. Replace any glow plugs that are not within specification to restore proper circuit operation.
- 4
Update PCM software or replace PCM
Check for available PCM software updates from the manufacturer and install if available. If communication issues persist after all other repairs, the PCM itself may require replacement.
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