P0684: Glow Plug Control Module to PCM Communication Circuit Range/Performance — what it means, the affected system and what to check before replacing parts.
Meaning, symptoms, causes and checks
Communication in the glow-plug control module-to-PCM communication circuit is detected, but its range, timing, plausibility or performance does not meet the monitor’s expectation. This range/performance definition does not by itself identify a failed module; power, ground, network integrity, message availability and vehicle-specific thresholds must be checked.
Affected system: Glow-plug module communication
Symptoms
- Warning light may illuminate
- Related system function may be impaired
- Vehicle-specific symptoms vary
Possible causes
- Implausible sensor or switch signal, poor ground, connector corrosion, or wiring damage
- Related wiring, connector, power or ground fault
- Vehicle-specific mechanical or control issue
What to check
- Record stored and pending codes and freeze-frame data
- Compare the named sensor or switch signal with actual operating conditions; inspect the connector, reference supply, ground and harness.
- Confirm measured values against the vehicle service information
- Retest under the conditions that set the code
- Compare the named sensor or switch signal with actual operating conditions; inspect the connector, reference supply, ground and harness.
- Repair the verified cause and confirm the code does not return
Also written as: P0684-00, P068400, PO684 (some scan tools add the fault-type byte, e.g. -00).