P0670 — Glow Plug Control Module Circuit — LLY / LBZ / LMM Duramax
Diagnostic reference for LLY/LBZ/LMM Duramax (2004.5–2010) P0670.
What it means
The PCM has detected a fault in the glow-plug control module circuit. On LLY/LBZ/LMM Duramax 2004.5-2010, glow plugs (8) + glow plug control module are used for cold start; there is no grid heater. Code P0670 applies to this platform.12
Most likely causes
- Glow plug control module failure — The dedicated glow plug control module is malfunctioning or has internal electrical fault. Typical on LLY/LBZ/LMM Duramax 2004.5-2010 with 8 glow plugs and a dedicated control module and no grid heater.1
- Wiring/connector corrosion or damage to glow plug control module circuit — Corrosion, chafing, or loose connections in the harness/connectors to the glow plug control module cause an open/short detected by PCM. Typical presentation for P0670 on these engines is wiring/connector corrosion or a short/open in the glow plug harness.1
- Short or open in glow plug harness / individual glow plug circuit — A short or open in the glow plug harness or at a glow plug can pull the control module circuit out of range. P0670 is described as a glow plug control module circuit malfunction often due to a failed module, wiring/connector corrosion, or a short/open in the glow plug harness on LLY/LBZ/LMM.1
How to diagnose
- Verify code P0670 is present and check for accompanying P067x glow plug codes; confirm platform is LLY/LBZ/LMM Duramax 2004.5-2010 with glow plugs + control module and no grid heater.
- Inspect glow plug control module connectors and harness for corrosion, moisture, oil contamination, and chafing; check power/ground feeds to the module. Typical causes include wiring/connector corrosion.
- Perform resistance/continuity checks of the glow plug control module circuit wiring from PCM to module and module to glow plugs; look for opens/shorts. P0670 typically involves wiring/connector corrosion or short/open in glow plug harness.
- Test glow plug control module command/response with a scan tool during cold-start enable; verify module powers the glow plugs and reports status. Module failure is a typical cause on this platform.
- If module tests good, isolate glow plug harness sections and check individual glow plug circuits for shorts/opens to ground or power that could affect the control module circuit. Short/open in glow plug harness is a listed typical cause.
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 GM enhanced diagnostics
- Terminal cleaning kit / dielectric grease
- Wiring diagram for LLY/LBZ/LMM glow plug control module circuit
Tools you’ll need
- OBD-II scan tool — Reads P0670 and accompanying P067x codes and can command the glow plug control module during cold-start enable to verify command/response.
- digital multimeter — Checks power/ground feeds to the glow plug control module and performs resistance/continuity tests for opens/shorts in the module-to-PCM and module-to-glow-plug wiring.
- inspection mirror and flashlight — Allows visual inspection of the glow plug control module connectors and harness for corrosion, moisture, oil contamination, and chafing in tight engine-bay locations.
- terminal cleaning kit — Cleans corroded or contaminated glow plug control module and harness connectors that commonly cause P0670 on LLY/LBZ/LMM Duramax.
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 control module replacement is required, have a qualified diesel technician perform diagnosis and repair to avoid electrical damage or incorrect cold-start operation. Do not attempt step-by-step repair of safety-critical systems. This guide is diagnostic only.
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 LLY / LBZ / LMM Duramax? This goes straight to the editor — it is not published.