P0380 on the 6.6L Duramax LLY is a glow plug circuit A malfunction. The Duramax uses glow plugs to warm the combustion chambers for cold starts, controlled by a glow plug control module (GPCM) that the ECM commands. P0380 sets when the module detects a problem in the glow plug feed circuit, such as an open, a short, or a plug drawing the wrong current.
Glow plugs do their most important work in the first few seconds of a cold start, warming the combustion chambers so the diesel lights off cleanly. On the LLY the ECM leans on the glow plug control module to manage that process and to report back when something is wrong. Because the plugs and the module age together, P0380 is often the first sign that the whole glow system is due for attention rather than a single isolated failure.
What P0380 means
The glow plug control module powers a bank of glow plugs and monitors current draw. When the ECM commands glow and the module sees the circuit outside its expected range, it reports P0380. This can mean one or more failed glow plugs, a wiring fault, or a problem with the control module itself.
Common causes
- One or more failed or open glow plugs
- Failed glow plug control module (GPCM)
- Corroded or loose glow plug feed connections
- Chafed or broken wiring between the module and plugs
- Poor ground or low battery voltage during glow cycle
Symptoms
- Hard cold starting and extended cranking
- White smoke on a cold start
- Rough running until the engine warms
- Check engine light with P0380 stored
- Wait-to-start behavior that seems ineffective
How to diagnose it
Start by testing each glow plug for resistance and continuity; a good plug reads a low resistance, while an open plug reads infinite. Compare all plugs to find the failed ones. Inspect the wiring and connectors from the control module to each plug for corrosion and damage. Confirm the module receives power and ground and commands the glow cycle with a scan tool. Verify battery voltage is healthy, since low voltage can disturb the current the module measures.
How to fix it
Replace any glow plugs that fail the resistance test, and it is common practice to replace them as a set for consistent cold starting. Repair corroded connectors and damaged wiring, and confirm good grounds. If the plugs and wiring are good but the code persists, replace the glow plug control module. After the repair, clear the code and confirm the glow system commands and draws current correctly, then verify an easy cold start. For glow plugs and control modules for your LLY, contact sales@coredieselparts.com.
Frequently asked questions
Will bad glow plugs keep my LLY from starting?
In warm weather the engine may still start, but in cold weather failed glow plugs cause hard starts, white smoke, and rough running until the engine warms.
Should I replace all the glow plugs at once?
It is a good idea. If one has failed, the others are the same age and often near the end of their life. Replacing the set restores consistent cold starting.
Is the code the plugs or the module?
Test the plugs first, since failed plugs are the most common cause. If all plugs and wiring check good, then suspect the glow plug control module.
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