U0106
Lost Communication with Glow Plug Control Module
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Code U0106 means the vehicle's engine control module has lost communication with the Glow Plug Control Module (GPCM). This code applies exclusively to diesel-powered vehicles, as glow plugs are used to preheat the combustion chambers in diesel engines to aid cold starting. The GPCM controls the timing and duration of glow plug operation and reports its status to the ECM via the CAN bus.
The most noticeable symptom is difficulty starting the engine in cold weather. Diesel engines rely on compression heat to ignite fuel, and when temperatures drop, glow plugs provide the extra heat needed for reliable ignition. Without GPCM communication, the glow plugs may not activate at all or may run for incorrect durations, leading to hard starts, excessive cranking, and white smoke from unburned fuel.
In warm weather, you may not notice much difference because the engine can start without glow plug assistance. However, the code should still be addressed because the GPCM also manages post-start glow plug operation that reduces emissions and improves idle quality. Check the GPCM fuse first—it carries high current and is a common failure point. Then inspect the connector, as the high-current pins can melt or corrode over time.
Severity
Symptoms
- •Diesel engine difficult to start, especially in cold weather
- •Glow plug indicator light not illuminating or staying on continuously
- •White or excessive smoke from exhaust during cold starts
- •Rough idle for extended period after cold start
- •Wait-to-start light not functioning properly
- •Check Engine Light illuminated
Likely Causes
The Glow Plug Control Module (GPCM) is mounted in the engine bay and subjected to extreme temperatures and vibration. Internal relay contacts, circuit board traces, or the CAN transceiver can fail, causing it to stop communicating.
The wiring harness connecting the GPCM to the CAN bus runs through the engine compartment where heat, moisture, and vibration can cause wire breakage, insulation degradation, or connector corrosion.
The GPCM draws substantial current to power the glow plugs. A blown high-current fuse, corroded power terminal, or poor ground connection will prevent the module from operating and communicating.
A short or open in the CAN bus wiring segment that includes the GPCM can prevent communication. If other modules on the same segment also show lost communication codes, the bus itself is likely the issue.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Check and replace the GPCM fuse, which handles high current for glow plug operation
- Inspect the GPCM connector for corrosion, melted pins, or loose connections
- Verify power supply voltage and ground at the GPCM connector
- Test CAN bus wiring continuity between the GPCM and the main harness
- Replace the glow plug control module if all wiring and electrical tests pass