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.
Low severity. Can I keep driving?You can keep driving, although cold starts will be harder. Avoid cranking the starter repeatedly on cold days.
Symptoms
- 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
Most likely causes
- Contact resistance or corrosion at connectors or pins of the communication line (signal distorted)
- Supply or ground problem at the glow module (voltage dip at high glow currents)
- Glow module faulty, or software/configuration not matching the engine module
- Engine module input stage faulty
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P0684 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 6: Control modules and outputs
- 84: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0684?
You can keep driving, although cold starts will be harder. Avoid cranking the starter repeatedly on cold days.
What causes code P0684?
The most common causes are: contact resistance or corrosion at connectors or pins of the communication line (signal distorted); supply or ground problem at the glow module (voltage dip at high glow currents); glow module faulty, or software/configuration not matching the engine module.
Related codes
- P0683Glow Plug Control Module 1 to PCM Communication Circuit
- P0670Glow Plug Control Module 1 Control Circuit/Open
- P064CGlow Plug Control Module 1
- P052FGlow Plug Control Module 1 System Voltage
- P06FDGlow Plug Control Module 2 to PCM Communication Circuit Range/Performance
- U0106Lost Communication with Glow Plug Control Module
Sources
- law.cornell.edu: 13 CCR 1968.2
- gsi.ext.gm.com: 18OBDG04 Engine Diagnostics
- gsi.ext.gm.com: 14OBDG10 Engine Diagnostics
- fordservicecontent.com: obd summary diesel 2023
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.