P0684: Glow Plug Control Module 1 to Powertrain Control Module Communication Circuit Range/Performance
What to do
Driveable, with hard cold starting. Mention any history of water reaching the engine bay, such as driving through deep standing water — these modules sit low and their connectors suffer for it.
What this code means
The glow plug module is communicating, but its messages do not make sense — reporting pre-heat that did not happen, or values outside the expected range. A plausibility fault rather than a lost connection.
A module with water in its connector produces exactly this, and glow plug modules are commonly mounted low in the engine bay where water reaches them.
Likely causes
Start here
- Failed glow plug control module · moderate
- Water ingress into connector · easy
- Corroded or loose connector · easy
Less common
- Damaged network wiring · specialist
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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