P0670

P0670 sets when the PCM loses communication or detects an electrical fault on the glow plug control module circuit, meaning the engine computer cannot command or verify glow plug operation. Start cheapest: this is a diesel-only code and cold-start hard-start complaints are the customer-facing symptom, so confirm hard-start before tearing into electrical diagnosis. Scan-tool check the glow plug module commanded state versus actual current draw if your tool supports it (typical glow plug current is 15 to 25 amps per plug on a 12V system during the pre-heat cycle), and verify battery voltage at the module's main power feed; a corroded power feed or melted fusible link is the cheap fix here. On platforms with a separate FICM or GPCM, check the ground straps and the small-gauge control wire from PCM to module; a single broken control wire mimics a failed module. Caveat: replacing all eight glow plugs and the harness on a P0670 without first verifying the module's power, ground, and PCM command signal is a $600 to $1,400 mistake.

Vehicle-specific patterns: 2003 to 2007 Ford 6.0L Powerstroke is the textbook P0670 vehicle, where the FICM and the GPCM both sit on the passenger valve cover and suffer water ingress from the cowl drain, corroding the connectors. 2008 to 2010 Ford 6.4L Powerstroke shares the same pattern with the dual-relay glow plug control. 2003 to 2009 Dodge Ram with the 5.9L and 6.7L Cummins uses a grid heater rather than glow plugs, but the grid heater relay sets P0670 when its terminals corrode. 2001 to 2010 GM 6.6L Duramax LB7/LLY/LBZ trips P0670 when one or more 12V glow plugs short to the head and pull the module into protection mode. Estimated repair: $120 to $1,400.