P0683Glow Plug Control Module to PCM Communication Circuit
Communication between the glow plug control module and the engine computer has been lost or corrupted on their dedicated data line.
Definition
On many diesels the PCM commands glow plug operation and receives per-plug fault feedback from a separate glow plug control module (GPCM) over a dedicated serial data circuit. P0683 sets when the PCM receives no response or invalid data from the GPCM. Glow plug control and diagnostics are then unreliable or lost entirely, producing hard cold starts, extended cranking, white smoke, and a MIL; individual plug fault codes may also stop reporting. This code is a well-known failure pattern on Ford Power Stroke diesels with a separate GPCM.
Common causes
- Failed glow plug control module (common failure)
- Open, shorted, or chafed wiring on the GPCM-to-PCM communication circuit
- Corrosion or water intrusion in the GPCM connector
- Blown fuse or poor power/ground supply to the GPCM
- PCM fault (rare)
Common misdiagnoses
- Replacing glow plugs when the plugs themselves are fine
- Replacing a glow plug relay on systems that use a module instead
- Battery replacement for the hard-cold-start symptom
- PCM replacement before checking module power, ground, and the data wire
Troubleshooting steps
1.Scan for companion codes
A U0106 (lost communication with GPCM) or multiple glow plug codes stored together support a module or supply-side failure.
2.Verify GPCM power and ground
Check the module's fuses and confirm battery voltage and a clean ground at the GPCM connector with the key on.
3.Inspect the module connector
Look for water intrusion, green corrosion, and backed-out pins — GPCMs are often mounted low where spray reaches them.
4.Test the communication wire
Check continuity of the data circuit between the GPCM and PCM connectors and verify it is not shorted to ground or power.
5.Substitute a known-good module
With power, ground, and the data line verified good, a persistent P0683 condemns the glow plug control module.