P0684Glow Plug Control Module to PCM Communication Circuit Range/Performance
Data passing between the glow plug control module and the engine computer is present but intermittent, implausible, or out of the expected range.
Definition
The PCM continuously validates the messages it exchanges with the glow plug control module on their dedicated communication circuit. P0684 sets when communication is established but the signal is erratic, corrupted, or fails plausibility checks, as opposed to P0683's complete loss of communication. Glow plug operation becomes inconsistent, causing intermittent hard cold starts, white smoke, and a MIL, and per-cylinder glow plug diagnostics may report unreliably. The fault is frequently intermittent and temperature- or moisture-dependent.
Common causes
- Intermittent wiring fault (chafed insulation or a loose terminal) on the communication circuit
- Corrosion or moisture in the GPCM or PCM connector causing signal degradation
- Failing glow plug control module with an erratic transceiver
- Poor GPCM ground causing signal offset
- Electrical interference from a damaged nearby high-current circuit
Common misdiagnoses
- Replacing glow plugs for what is a data-circuit fault
- Replacing the GPCM before finding an intermittent harness problem that then damages the impression the new module fixed it
- Battery or alternator diagnosis for the intermittent cold-start complaint
- PCM replacement
Troubleshooting steps
1.Document freeze frame and conditions
Note temperature and moisture conditions when the code sets, since range/performance communication faults are usually intermittent.
2.Wiggle-test the communication circuit
Monitor GPCM data with a scan tool while flexing the harness between the GPCM and PCM to expose an intermittent open.
3.Inspect both connectors
Check the GPCM and PCM connector cavities for corrosion, spread terminals, and evidence of water entry.
4.Verify module ground integrity
Voltage-drop test the GPCM ground under load; more than about 0.1 V drop can corrupt serial data.
5.Replace the GPCM only after wiring passes
A module that still sends invalid data over a verified-good circuit has failed internally.