P0266

P0266 means the ECM detected cylinder 2 contribution/balance fault — same diagnostic framework as P0263 but on cylinder 2. The ECM measures crankshaft acceleration per firing event and per-cylinder injector balance rate, then flags cylinder 2 when its mm^3/stroke deviation pushes past the 2-5 mm^3 window or its contribution to crankshaft acceleration drops below roughly 85-92% of bank average. Cheapest-first diagnostic ladder: (1) scan-tool the per-cylinder injector balance rate at warm idle — a cylinder 2 reading +/-4 mm^3/stroke or worse with the other cylinders within +/-1 mm^3 is the giveaway; positive deviation means the injector is over-fueling (stuck/dribbling) and the ECM is pulling fuel away, negative means under-fueling (clog/wear) and the ECM is adding fuel to compensate. (2) Swap the cylinder 2 injector electrical connector with cylinder 3 (or any adjacent known-good cylinder), clear codes, and watch whether the balance fault migrates to the new cylinder — this isolates electrical from mechanical/hydraulic in 10 minutes. (3) Return-flow test the injectors with calibrated bottles (target less than 50-100mL/min per injector at idle on most HPCR systems; a cylinder returning 200+ mL/min is stuck open). (4) Relative compression test via scan tool starter-current monitor — variation greater than 15% from the highest-reading cylinder is mechanical (worn rings, valve leak, head gasket). (5) On platforms with injector trim codes (most modern HPCR), confirm the trim/IQA code in the ECM matches the code etched on the injector — a swapped injector without recode causes balance faults all on its own. Expensive-misdiagnosis caveat: don't replace just the cylinder 2 injector on a high-mileage diesel — once one HPCR injector fails the rest are typically 80% to failure and the labor to drop and replace one (especially on LB7 Duramax under-the-valve-cover designs) is the same as doing all of them. Replacing one and being back in the bay in 30 days for cylinder 3 is a common shop-floor regret.

Vehicle-specific patterns: 2001-2004 GM Duramax LB7 throws P0266 along with the rest of the cylinder-balance family because the LB7 injectors fail as a set under the valve cover — GM's extended 200k-mile warranty covered many; out of warranty, plan on a full-set replacement at $3,500-6,000. 2003-2007 Ford 6.0L Powerstroke throws P0266 from HEUI injector stiction or from low high-pressure oil pump output starving the injector hydraulically; IPR regulator and HPOP health must be confirmed before condemning injectors. 2008-2010 Ford 6.4L Powerstroke throws P0266 from Bosch piezo injector wear. 2011-2016 Ford 6.7L Powerstroke throws P0266 from CP4 contamination metal damaging injector nozzles. 2003-2018 RAM Cummins 5.9L/6.7L throws P0266 from Bosch CR injector wear; the inline-6 makes injector replacement relatively quick at ~6-8 hours labor. 2007-2018 Mercedes Sprinter OM642 3.0L V6 throws P0266 from piezo injector wear and from injector trim mismatch after replacement without coding. 2009-2014 Audi/VW TDI 2.0L throws P0266 from injector nozzle wear and CP4 metal contamination. Estimated repair: $400 to $7,000.