OBD-II code · Powertrain (generic)
P0684 Glow Plug Control Module 1 to PCM Communication Range or Performance
Same as P0683 with the communication signals in plausible range yet not matching expected protocol or content.
Is it safe to drive with P0684?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyProtocol mismatch
- Less likelyGateway module routing CAN messages incorrectly
- Less likelyCAN terminating resistor (120 Ω) drifted out of spec
Symptoms
- Glow plug warning light on (often orange coil symbol)
- Hard cold start or no start in cold weather
- White smoke from exhaust during warm-up
Repair at a glance
- Difficulty
- Medium
- DIY friendly
- Best left to a mechanic
- Typical labour
- 0.5–2 h
- Emissions test
- Likely to fail
- Parts involved
- Glow Plug Control Module 1
Related codes
- P06FDGlow Plug Control Module 2 to PCM Communication Range or Performance
- P0681Cylinder 11 Glow Plug Circuit Open
- P0682Cylinder 12 Glow Plug Circuit Open
- P0683Glow Plug Control Module 1 to PCM Communication Circuit
- P0685ECM or PCM Power Relay Control Circuit Open
- P0686ECM/PCM Power Relay Control Circuit Open
- P0687ECM/PCM Power Relay Sense Circuit Open
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.