P0679

P0679 means the ECU detected a glow plug circuit fault on cylinder 8 — open, short, or resistance out of spec from the GPCM monitor. The cheapest-first diagnostic ladder: (1) Ohm the cyl 8 plug at its terminal — healthy 0.4-2.0 ohms cold (0.6-1.2 typical), 0 shorted, infinite open, 4+ weak. (2) Confirm GPCM PWM command at 0-100% with glow-on time 5-30 sec scaled to coolant temp. (3) Amp probe on the cyl 8 feed should show 7-20A initial draw falling to 5-15A; flat 0A with command ON is an open, sustained 20A+ is a short. Cylinder 8 only exists on V8, V10, or V12 diesels — in modern shop work that's a V8 Powerstroke or Duramax — and cyl 8 is universally the rearmost cylinder on one bank, which means the worst access on the engine. 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: 2003-2007 Ford 6.0L Powerstroke V8 throws P0679 from cylinder 8 (passenger bank, rearmost) — cyl 8 lives under the cowl behind the alternator and is the single hardest plug to service on the engine; if it's seized, this is often the cylinder that pushes the job to a full head pull. Plug-seizure rates on 6.0 cyl 7/8 are high enough that experienced shops quote head removal as a contingency on every P0678/P0679 job. 2008-2010 Ford 6.4L Powerstroke throws P0679 from the under-valve-cover harness wear-through at the rear rocker. 2001-2016 Duramax V8 throws P0679 on cyl 8 (passenger bank rearmost) — Duramax plug access is better than Powerstroke but the GPCM mounting position on the passenger inner fender means heat-cycle damage to the cyl 8 driver inside the module is also common; if the plug ohms good but amp draw is 0, swap the GPCM. 2008-2018 Ford 6.7L Scorpion Powerstroke throws P0679 from harness corrosion. Estimated repair: $150 (single plug with extended labor) to $3,500 (head pull for seized cyl 8 plug on the 6.0).