P0684Glow Plug Control Module 1 to PCM Communication Circuit Range/Performance
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- Glow plug module 1 reports to the engine module over a feedback wire (Bosch-type systems expect it inverse to the activation line) or the bus; GM and Ford report a lost bus link as U0106, Saab as P0380/P1380, and no manufacturer document available to us uses P0683/P0684 themselves. P0684 means communication exists but is implausible: wrong level or duty cycle, implausible content or a feedback that does not match the activation state – as in the Saab logic, where the feedback must be the inverse of the command. The engine module can then no longer rely on the glow system status; individual plug faults may go unreported. Contact resistance at the pins and voltage dips at the glow module during high glow currents are typical causes.
- What you may notice
- Check engine light (glow plug lamp may flash or stay on)
- Harder cold start, long cranking in frost
- Rough running and white smoke after a cold start
- No individual plug reports any more
- Likely causes
- LikelyContact resistance or corrosion at connectors or pins of the communication line (signal distorted)
- PossibleSupply or ground problem at the glow module (voltage dip at high glow currents)
- PossibleGlow module faulty, or software/configuration not matching the engine module
- RareEngine module input stage faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €80 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.