Glow Plug Control Module to PCM Communication Circuit Range/Performance
Your engine's glow plug control module isn't communicating properly with the main computer, like a radio with weak signal that keeps cutting out. The PCM can't receive consistent data about glow plug readiness, especially on diesel engines during cold starts.
The PCM monitors voltage and signal timing from the glow plug control module over a dedicated communication circuit, expecting stable handshake signals within specific timing windows. It checks for signal dropout, voltage anomalies, and response delays that indicate a degraded or failing communication link.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Module Response Time | 10-50 milliseconds | >100ms or no response |
| Signal Voltage | 4.5-5.5 volts | <3.0V or >5.8V |
Code P0684 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0684 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.