P0670: How to Diagnose & Fix — Glow Plug Module Control Circuit
2 vehicles · 2 makes · $250-$1,800 repair
How to diagnose and fix P0670: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
GMC Sierra 3500HD · 2007–2019
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)
Chevrolet Express · 2011–2016
The 6.6L Duramax LGH offered in Express 2500/3500/4500 vans from 2011 to 2016 has a well-documented glow plug system weakness. Individual glow plugs fail open, and the glow plug control module itself develops open-driver and no-power-to-glow-plug faults - a failure mode common enough that specialist board-repair houses list the 2011-2016 Chevrolet Express 6.6L LGH GPCM as a standing repair line. GM issued a service bulletin covering 2011-2012 Express vans with the LGH and LML Duramax where the malfunction indicator lamp illuminates for glow plug faults; the bulletin notes there is usually no drivability complaint in mild weather, but harder cold starting may be present. In practice owners in cold climates get extended cranking, white startup smoke and a rough first minute of running, plus a persistent MIL that will fail an inspection. Do not confuse this with the 2017-2022 Express, which offered the completely different 2.8L Duramax four-cylinder; the diesel was dropped for 2023.
Common Symptoms
- Malfunction indicator lamp on with glow plug circuit codes
- Extended cranking in cold weather
- White smoke on cold startup
- Rough running for the first minute after a cold start
- Wait-to-start behavior that never satisfies
How to Fix
Scan for the specific glow plug circuit code to identify which cylinders report open, then load-test each glow plug individually - a single open plug sets the light, so replacing the module first is a common misdiagnosis. If the module reports open drivers or no output at all, the GPCM is at fault and can either be replaced new or sent for board-level repair at a fraction of dealer cost. When multiple plugs test bad, replace the full set; mixing old and new plugs invites a repeat visit. Access is tight - budget significant labor, particularly on the driver's side in the van body. No recall covers this. GM special coverage 15114 covered the glow plug on 2011-2012 Express vans for 10 years/120,000 miles, but that coverage has now expired by age.
Sources for this issue (4)
- GM Special Coverage Adjustment 15114 - MIL Lamp On / Glow Plug Heater Inoperative (2010-2012 Express/Savana 6.6L LML, LGH)tsb
- 2011-2016 Chevrolet Express 2500/3500/4500 6.6L Duramax LGH Glow Plug Control Module Repair - documented failure modes: no power to glow plugs, open driver errorsarticle
- $4,000 Duramax glow plug problem "not uncommon" per Chevrolet Customer Service - Chevy and GMC Duramax Diesel Forumforum
- Glow plugs going bad - Chevy and GMC Duramax Diesel Forumforum
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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?
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.
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?
Repair costs for P0670 range from $250 to $1,800, depending on the vehicle and root cause.
Which vehicles are affected by P0670?
Au7o has documented P0670 across 2 vehicle models from 2 manufacturers: Chevrolet, GMC.
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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.