P0278

P0278 means the cylinder-balance monitor flagged cylinder 6 for contributing outside the +/- 5 mm3/stroke deviation window the ECM allows on per-cylinder fuel trim. Diagnostic flow is identical to any contribution/balance code, but cylinder-6 location matters: on most domestic V8s (Ford modular, GM LS, Chrysler Hemi) cylinder 6 is on the left (driver) bank, second from the front; on a Ford Powerstroke 6.0L/6.4L/6.7L V8, the bank assignments follow the standard 1-3-5-7 right / 2-4-6-8 left layout; on a Duramax LB7-LML 6.6L V8, cylinder 6 is on the left bank rear; on a Cummins inline-6, cylinder 6 is the rear-most cylinder closest to the firewall. Cheapest-first ladder: pull live cylinder balance data, look for cyl 6 deviation past +/- 5 mm3, then run a manual cylinder cutout test (disable cyl 6 injector and watch RPM drop — a healthy cylinder drops idle RPM by 80-150 RPM; a weak cylinder drops less than 50 RPM). Next: glow plug current draw on diesels (a shorted glow plug skews thermal balance), compression test (looking for less than 85% of highest), injector return-flow test (back-leak test on common-rail diesels — a leaking injector returns 3-10x normal fuel volume to the tank). Expensive-misdiagnosis caveat: on inline-6 diesels especially, a cracked or eroded injector cup in the cylinder head will exactly mimic a failing injector and cost $1,800-$3,500 in head-pull labor to find after you've already replaced the injector.

Vehicle-specific patterns: 1998-2007 Dodge RAM 2500/3500 with the 5.9L Cummins inline-6 commonly throws P0278 on cylinder 6 specifically because it's the rear-most cylinder and sits closest to the turbo + firewall heat — injector heat-soak failure or a #6 injector cup leak (the cup is a brass sleeve that isolates the injector tip from coolant) is the classic finding. 2008-2010 Ford 6.4L Powerstroke V8 throws P0278 from the well-documented piston cracking and EGR-cooler failure that contaminates injectors — Ford issued multiple TSBs and the affected cylinders cluster on the right bank. 2007-2010 Duramax LMM 6.6L throws P0278 from injector body cracks at high mileage that bleed rail pressure. 2011-2018 BMW N57 / B57 inline-6 diesel (European market and limited US) throws P0278 from EGR-cooler carbon contamination starving the rear cylinders of clean intake air. Estimated repair: $200 (single glow plug + injector clean) to $5,500 (head pull and injector cup replacement on a Cummins).