P0304

P0304 means cylinder 4 is misfiring. The 10-15 minute coil swap drill is the same: pull cylinder 4's plug, read it, swap its coil with an adjacent cylinder, clear codes, and confirm whether the misfire moves. On a 4-cylinder engine, cylinder 4 is the cylinder furthest from the timing-belt/chain end and often runs slightly hotter and leaner because of intake-runner length and exhaust heat soak from the manifold collector. On a transverse V6 (Honda J-series, Toyota 2GR, Chrysler Pentastar), cylinder 4 typically lives on the front bank closest to the radiator, which means it's also the easiest cylinder to access — useful when the swap test points back to it. If the coil-swap and plug-swap both fail to move the misfire, run a compression test, then a leak-down test if compression is borderline (15-20% leakage past the rings is wear; >25% leaking out the exhaust is a burned valve).

Vehicle-specific patterns: 2002-2006 Toyota Camry/Highlander with the 2AZ-FE 2.4L is famous for P0304 from oil consumption — the piston rings glaze, oil enters cylinder 4 specifically because of its position relative to the PCV pickup, and fouls the plug repeatedly; Toyota issued a TSB for ring replacement on affected VINs. 2011-2016 Chevy Cruze/Sonic with the 1.4L turbo (LUJ/LUV) frequently throws P0304 from a cracked intake manifold or failed PCV diaphragm causing a lean condition that hits cylinder 4 hardest. 2007-2013 Mazda 3/CX-7 with the 2.3L DISI turbo throws P0304 from carbon-fouled VVT solenoids retarding cam timing under load. Estimated repair: $40 (coil/plug) to $3,500+ (ring job on the 2AZ-FE).