P0677

P0677 means the ECU detected a glow plug circuit fault on cylinder 6 — open, short, or resistance out of spec per the GPCM. The cheapest-first diagnostic ladder: (1) Ohm the cyl 6 plug at its terminal — healthy 0.4-2.0 ohms cold (most diesels 0.6-1.2 ohms), 0 shorted, infinite open, 4+ weak. (2) Verify the GPCM is commanding cyl 6 at the expected PWM duty (0-100%) with glow-on time 5-30 sec scaled to coolant temp. (3) Amp probe on cyl 6 feed should show 7-20A initial draw falling to 5-15A as plug heats; flat 0A with command ON = open circuit, sustained 20A+ = short. Cylinder 6 implies inline-6, V6, V8, or V10 architecture. 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 P0677 from cylinder 6 (passenger bank, third from front) — same EGR-cooler proximity as cyl 5; cyl 6 is also where the FICM injector driver harness frequently chafes through. 2008-2010 Ford 6.4L Powerstroke throws P0677 from the under-valve-cover glow plug harness fault. 2001-2016 Duramax V8 throws P0677 on cyl 6 (passenger bank) — LMM and LML are slightly more durable than LB7/LLY but still throw the code on plug fatigue around 100k miles. 2007-2018 Cummins 6.7 inline-6 throws P0677 on the rearmost cylinder of the block (cyl 6, right against the firewall) — this is the hardest plug to reach on the 6.7 because the cab tilts forward only on Class 7+ trucks; on Dodge/RAM 2500/3500, expect 1.5 hr labor just to access cyl 6. 2007-2018 Mercedes Sprinter OM642 V6 uses cyl 6 as the rearmost passenger-bank cylinder and routinely throws P0677 from poor cooling around the turbocharger heat shield. Estimated repair: $120 (single plug with extended labor) to $1,600 (full set on the Cummins).