P0671 on GMC: How to Diagnose & Fix
Cylinder 1 Glow Plug Circuit
1 GMC model · 1 documented issue · $250-$1,200 repair
P0671 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 (1)
P0671 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 P0671 mean on a GMC?
P0671 stands for "Cylinder 1 Glow Plug Circuit." This is a diesel engine code. Glow plugs are small heating elements that warm each cylinder's combustion chamber so a diesel engine can start easily, especially in cold weather. P0671 means the engine computer (through the glow plug control module) detected an electrical fault in the circuit for the cylinder 1 glow plug — typically an open circuit, a short, or resistance that's out of the expected range. The most noticeable effect is harder cold starting, rough running until warm, or increased white exhaust smoke on startup. On GMC it is documented across 1 model: Sierra 3500HD.
What are the most common causes of P0671?
The most common causes of P0671 are: Failed (open or shorted) cylinder 1 glow plug, Damaged or corroded glow plug wiring/connector, Loose or corroded glow plug terminal/strap, Faulty glow plug relay or control module, Broken or chafed harness to the glow plug, Poor ground or supply voltage to the glow plug circuit. The specific cause varies by vehicle.
How much does it cost to fix P0671 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.
Looking for P0671 on a different make?
View P0671 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.