P0670 — Glow Plug Control Module Circuit — LB7 Duramax
The PCM has detected a fault in the glow-plug control module circuit on the LB7 Duramax. This is a cold-start system fault; the Duramax uses glow plugs with a…
What it means
The engine control module has detected an electrical fault in the glow-plug control module circuit. On the LB7 Duramax, cold start uses eight glow plugs controlled by a glow plug control module (GPCM).1
Most likely causes
- Glow plug control module (GPCM) internal failure or power/ground loss — The PCM monitors the GPCM control circuit; loss of power, ground, or internal module failure sets P0670.1
- Open or short in GPCM harness/connectors — Chafed or corroded wiring to the GPCM can cause the PCM to see an out-of-range circuit condition.1
- Related glow-plug heater circuit malfunction — A broader glow-plug/heater circuit fault such as P0380 can accompany or contribute to a GPCM circuit fault on this platform.2
How to diagnose
- Verify freeze frame and confirm P0670 is present with cold-start symptoms.
- Inspect GPCM power and ground feeds and connector for corrosion, moisture, or damage.
- Check GPCM harness for opens/shorts to ground or battery; compare to wiring diagram.
- Command GPCM activation with a scan tool and monitor GPCM feedback/state PID if available.
- Inspect glow plug supply wiring and individual glow plug resistance for shorts/opens that may load the GPCM circuit.
- Clear codes, cold soak, and retest for P0670 return.
Specs and test values
| Check | Normal reading | When to measure | Source |
|---|---|---|---|
| Battery and charging voltage | 12.6–12.7 V at rest when fully charged; about 13.5 V running | Engine off, then running with loads on | DieselHub |
| Glow plug resistance | Low — usually no more than about 6 Ω — and similar on every plug; an open reading means a failed plug | Engine off, meter lead to engine ground | DENSO |
Common mistake to avoid
Replacing glow plugs or the glow module before checking the batteries. The glow system draws heavily, and low battery voltage causes circuit faults that no part swap will fix. Source: FleetRabbit — P0670 glow plug control module fault.
Parts and tools you may need
- Digital multimeter
- Scan tool with bi-directional control and live data
- Wiring diagram for LB7 GPCM circuit
- Dielectric grease
- Basic hand tools
Tools you’ll need
- OBD-II scan tool — Read P0670, view freeze frame, and command GPCM activation to verify module response during cold start.
- digital multimeter — Check GPCM power, ground, and harness continuity for opens/shorts and measure glow plug resistance to load the circuit.
- wiring diagram — Trace GPCM power/ground and glow plug circuits to pinpoint opens/shorts and connector pinouts.
- inspection light — Visually inspect GPCM connector and harness for corrosion, moisture, or damage in the engine bay.
Rough cost and difficulty
| Likely repair | Parts | Shop labor | Who usually does it |
|---|---|---|---|
| Wiring or connector repair | $10–$100 | 1–4 hrs | DIY with tools and a manual |
| Glow plugs (full set) | $80–$250 | 2–6 hrs | DIY with tools and a manual |
| Glow plug relay or control module | $60–$400 | 0.5–2 hrs | Easy DIY |
Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.
When to see a pro
If wiring faults are found in high-current glow-plug circuits, or if GPCM replacement is required, have a qualified diesel technician perform the work to avoid electrical damage or incorrect cold-start operation. Safety-critical high-current circuits should not be serviced without proper training and tools.
Check for recalls
Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.
Sources
Spotted an error, or something missing? Tell us
Run a LB7 Duramax? This goes straight to the editor — it is not published.