P0684 code meaning
Glow Plug Control Module 1 to PCM Communication Circuit Range/Performance
Powertrain system Generic code P06xx codes Check engine light
What P0684 means
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.
P0684 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of P0684
- 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
Common causes of P0684
- Most 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
- Less likelyEngine module input stage faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Glow plug control moduleGlow plug feedback lineEngine control module
What each part of P0684 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 6 | Area: computer (engine control module) and output circuits. |
| 84 | The exact fault in that area. |
What to do next
- Note P0684 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0684
What does P0684 mean?
P0684 means "Glow Plug Control Module 1 to PCM Communication Circuit Range/Performance". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P0684?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0684 fail an emissions test?
Yes, it can. P0684 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0684 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P0684 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.