P0670 on GMC: How to Diagnose & Fix
Glow Plug Module Control Circuit
1 GMC model · 1 documented issue · $250-$1,200 repair
P0670 on GMC by model: symptoms, diagnosis and fix
Duramax-equipped Sierra 3500HD owners report hard starting, extended cranking, and rough idle in cold weather due to glow plug system faults. NHTSA complaints and service information commonly involve failed glow plugs, glow plug control module issues, or wiring/connector problems that trigger circuit DTCs and reduce preheat performance.
Common Symptoms
- Hard start or no-start in cold temperatures
- Extended cranking time
- Rough idle and white/gray smoke on startup
- Check Engine light with glow plug circuit codes
- Reduced power briefly after cold start
How to Fix
Scan for glow plug and control module DTCs, load-test the glow plugs, and inspect harness/connectors at the glow plug controller. Replace failed glow plugs and/or the glow plug control module as indicated, then clear codes and verify proper preheat operation. Use OEM glow plugs specified for the engine (LMM/LML/L5P vary by year).
Sources for this issue (2)
P0670 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0670 mean on a GMC?
P0670 stands for "Glow Plug Module Control Circuit." This code indicates the engine computer detected a fault in the control circuit between the PCM and the glow plug control module on a diesel engine. The glow plug module energizes the glow plugs to preheat the cylinders for cold starting. A fault here mainly causes hard cold starts, rough cold-start running, and white smoke until the engine warms up. On GMC it is documented across 1 model: Sierra 3500HD.
What are the most common causes of P0670?
The most common causes of P0670 are: Faulty glow plug control module, Open or shorted wiring between PCM and glow plug module, Corroded or loose module connectors, Blown fuse feeding the glow plug system, Poor ground at the module, PCM control circuit fault. The specific cause varies by vehicle.
How much does it cost to fix P0670 on a GMC?
Published repair estimates on GMC range from $250 to $1,200 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.