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 PCM detects that the glow plug control module is not responding or is sending incorrect signals over the communication circuit. This can lead to improper glow plug operation, affecting cold starts and engine performance, especially in diesel engines.
01. Most Common Causes
- Faulty glow plug control module
- Open or shorted communication circuit wiring
- Corroded or loose connectors
- Failed PCM (rare)
02. Symptoms You Will Notice
- Hard starting in cold weather
- Check Engine Light illuminated
- Excessive white smoke on startup
- Reduced engine power or limp mode
03. Step-by-Step Diagnosis
Step 1: Scan for additional codes and check freeze frame data.
Step 2: Visually inspect glow plug control module connector and wiring for damage or corrosion.
Step 3: Test communication circuit continuity and resistance between glow plug module and PCM.
Step 4: Check power and ground supplies to glow plug control module.
Step 5: If all tests pass, replace glow plug control module and clear codes.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive long distances, especially in cold weather, as it may cause hard starting and potential stalling. Have it diagnosed promptly.
How much does it cost to fix?▼
Typical repair costs range from $50 to $150 for diagnostic and module replacement, but can vary based on labor rates and parts.