P0674

P0674 means the ECU detected a fault in the glow plug circuit feeding cylinder 3 specifically — open, short, or resistance out of spec per the Glow Plug Control Module's per-cylinder current monitor. The cheapest-first diagnostic ladder: (1) Disconnect the glow plug pigtail at cylinder 3 and ohm the plug at the terminal — healthy reading is 0.4-2.0 ohms cold (most plugs land 0.6-1.2 ohms); 0 ohms is a dead-shorted plug, infinite is an open/burned filament, and 4+ ohms is a weak plug on its way out and is the leading cause of hard cold-start complaints on diesels with no DTC at all. (2) Verify the GPCM PWM command on the scan tool — duty cycle 0-100% scaled to coolant temp, glow-on time 5-30 sec; if the GPCM is commanding 0% on cylinder 3 only, the module logic has flagged the circuit as faulted and locked it out. (3) Clamp an amp probe on the cyl 3 feed wire and confirm 7-20A initial inrush falling to 5-15A as the plug heats; 0A with command ON is an open in the harness or a burned plug. Test resistance INDIVIDUALLY at the glow plug terminal — pulling a shorted plug measures 0 ohms; an open plug measures infinite; a marginally weak plug measures 4+ ohms cold and is the most common cold-start hard-start root cause. Replace as a set on diesel engines — mixed-age plugs cause progressive failure.

Vehicle-specific patterns: 2008-2010 Ford 6.4L Powerstroke V8 throws P0674 from cylinder 3 (driver's bank, second from front) when the under-valve-cover glow plug harness chafes against the rocker assembly — Ford issued multiple service bulletins on the 6.4 UVCH (under-valve-cover harness); cylinder 3 is one of the most-replaced positions because of its proximity to the EGR plumbing. 2011-2016 Duramax LML/LGH 6.6 V8 throws P0674 from a known glow plug failure mode where the tip swells and either breaks off into the cylinder (catastrophic) or seizes in the head — GM TSB territory; if the plug won't unscrew at 30 ft-lb, stop and soak with penetrant for 24 hr before twisting it off. 2007-2018 Cummins 6.7 inline-6 throws P0674 on cylinder 3 (mid-block, the hottest position thermally) more often than any other cylinder. 2009-2014 Mercedes Sprinter OM642 V6 throws P0674 when one bank's glow plug harness gets pulled tight during turbo or EGR service. Estimated repair: $80 (single plug, accessible) to $2,800 (broken-off tip extraction on the LML).