P0271

P0271 sets when the cylinder 4 injector driver circuit is detected at high voltage when it should be pulled low during the injection pulse, mechanically identical to P0262/P0265/P0268 but on cylinder 4, which on inline-4 engines is the cylinder closest to the firewall and on most V6/V8 platforms is on the passenger bank. The diagnostic ladder starts cheap: ohm the injector at the connector (11.4-18 ohms PFI, 1.5-4 ohms GDI), verify 12V on the supply side with key-on, then scope the driver line for the inductive flyback spike on the trailing edge of the pulse. If the injector ohms in spec but the harness shows open, perform a wiggle-test along the loom and inspect for chafe at any bracket pass-through or exhaust-adjacent run. Pin-drag test the connector terminals, terminal back-out from worn retention tabs is a high-frequency root cause on engines with 150K+ miles. Expensive misdiagnosis: on transverse 4-cylinder platforms cylinder 4 is the firewall-side cylinder and replacement requires either pulling the intake or working blind around the bulkhead, do not pull the intake until you have positively confirmed the injector itself is the fault by ohming it out of the harness, because the actual cause on aged vehicles is more often a corroded connector pigtail than a failed injector.

Vehicle-specific patterns: 2002-2006 Nissan Altima/Sentra 2.5L QR25DE has a chronic cylinder 4 injector pigtail problem where heat soak from the rear exhaust manifold cracks the connector body; 2007-2012 Mazda CX-7 2.3L DISI throws P0271 when the direct-injection high-pressure injector solenoid opens internally, the injectors are not individually serviceable on early units; 2010-2015 VW/Audi 2.0L TSI (EA888) sees cylinder 4 injector failures from carbon buildup loading the winding and from harness chafe at the firewall pass-through, TPI 2026975 documents the replacement procedure. Estimated repair: $240 to $1,300.