Glow Plug Control Module 1 to PCM Communication Circuit Range/Performance
What it means
P0684 is a diesel code meaning the PCM sees a range or performance problem in the communication circuit between it and the glow plug control module — the two are talking, but the data looks implausible or intermittent. Because that link governs cold-start pre-heating, a fault here can make cold starts hard and smoky. It usually points at the wiring, connectors, or the glow-plug module rather than the engine.
Can I keep driving? The engine usually still runs, but the PCM and glow-plug module are not exchanging data reliably, so cold-start pre-heating may be off until it is fixed.
Common symptoms
What you'll notice
- Check-engine light on (often with a glow-plug warning light)
- Hard or slow cold starts
- Rough running and white smoke on a cold start
- Glow-plug indicator behaving oddly (staying on or not lighting)
Common causes
Most likely first
- Damaged or corroded wiring in the communication line between the glow-plug module and PCMLikely
- Poor connector or ground at the glow plug control moduleCommon
- Faulty glow plug control module sending implausible dataCommon
- Failed PCMLess often
How to diagnose & fix
Work through it in order
- Scan and record freeze-frame data; note any companion glow-plug codes.
- Inspect the communication wiring and connectors between the glow-plug control module and PCM for corrosion, damage, or loose pins.
- Verify the module's power and ground feeds are clean and tight.
- If wiring and feeds check out, replace the glow plug control module; consider the PCM only after the module and harness are ruled out.
FAQ
Quick answers
Is it safe to drive with P0684?+
Can I just clear P0684?+
Related codes
Often seen with P0684
Read & clear P0684 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.