P0670 — Glow Plug Control Module Circuit — L5P Duramax
Diagnostic reference for P0670 on L5P Duramax (2017–present).
What it means
The PCM has detected a fault in the glow-plug control module circuit. On L5P Duramax, cold start uses glow plugs (8) + glow plug control module; NOT a grid heater.1
Most likely causes
- Glow plug control module internal fault or power/ground loss — Module not responding or power/ground circuit open/short; PCM sets P0670 for control-module circuit fault.1
- Wiring harness chafing / connector corrosion at glow plug control module — Open/short in module harness triggers control-module circuit DTC.1
- Repeated individual glow plug failures stressing the control module — L5P glow plug failures are reported; repeated failures can lead to module circuit faults.2
How to diagnose
- Verify freeze frame and confirm P0670 is present with glow-plug system active (cold start conditions).
- Inspect glow plug control module connectors for corrosion, bent pins, and harness chafing; check module power and ground circuits for continuity and proper voltage.
- Command glow plug system with scan tool and monitor module communication/status; look for module response failure indicating internal fault.
- Check related glow plug circuit codes (e.g., P0380, P067x) to identify if individual plug faults are contributing.
- If module tests good, inspect individual glow plug circuits for high resistance/short that may overload module; document findings before replacement.
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 for glow plug system
- Terminal cleaning kit
- Dielectric grease
Tools you’ll need
- OBD-II scan tool — Commands glow plug system and reads P0670 freeze frame plus related P067x/P0380 codes to confirm module communication failure.
- digital multimeter — Checks module power, ground continuity, and harness voltage drop to rule out wiring faults before condemning the control module.
- glow plug circuit tester — Tests individual glow plug resistance and current draw to identify overloaded plugs stressing the L5P glow plug control module.
- inspection mirror and LED flashlight — Reveals harness chafing and connector corrosion at the glow plug control module in tight engine bay areas.
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 diagnosis points to glow plug control module replacement or internal harness repair, have a qualified diesel technician perform the work. Glow plug service on high-compression diesel requires proper torque and electrical safety practices.
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
Repair videos: P0670 on the L5P Duramax
Third-party videos, not ours — picked for this engine. Confirm anything against the factory service information for your truck.
Spotted an error, or something missing? Tell us
Run a L5P Duramax? This goes straight to the editor — it is not published.