P0676

P0676 means the ECU detected a glow plug circuit fault on cylinder 5 — open, short, or out-of-spec resistance flagged by the GPCM per-cylinder monitor. The cheapest-first diagnostic ladder: (1) Ohm cylinder 5's 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% duty cycle with glow-on time 5-30 sec scaled to coolant temp. (3) Clamp an amp probe on the cyl 5 feed and verify 7-20A initial inrush falling to 5-15A as the plug heats — 0A is open, sustained 20A+ is short. Cylinder 5 only exists on 5-cylinder, 6-cylinder, 8-cylinder, or 10-cylinder diesels — typically inline-6 (Cummins, OM606), V6 (OM642 Sprinter), V8 (Powerstroke, Duramax), or V10 (Touareg TDI). 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 P0676 from cylinder 5 (driver's bank, third from front) — cyl 5 sits directly under the EGR cooler on the 6.0 and is the cylinder most likely to seize a glow plug into the head due to coolant seepage from cracked EGR cooler internals; if the plug won't budge, soak for 48 hr and use a torque-limited extractor. 2008-2010 Ford 6.4L Powerstroke commonly throws P0676 from the under-valve-cover harness rubbing through at the rocker shaft. 2001-2016 Duramax V8 throws P0676 on cylinder 5 (driver's bank) — the LB7 and LLY are especially prone because their factory plugs were a known weak design. 2007-2018 Cummins 6.7 inline-6 commonly throws P0676 on the rear-of-block cylinders (5 and 6) where the firewall traps heat. 2007-2018 Mercedes Sprinter OM642 V6 BlueTec uses cyl 5 on the rear bank — the V6 layout means cyl 5 is harder to access than cyl 1-4. Estimated repair: $80 (single plug, accessible) to $3,200 (seized-plug extraction with head removal on the 6.0).