P0312

P0312 is cylinder 12 misfire detected — V12 only, and almost always the very last cylinder in the firing order on the right bank, physically tucked against the firewall on FR V12 platforms. The mechanical diagnostic ladder is identical to P0311: swap coil, inspect plug, swap injector, compression test, leak-down. The labor reality is brutal — on a 760Li the cylinder 12 coil is under the intake plenum at the rear corner, requiring 1.5-2 hours of disassembly to reach a $90 part. Before tearing into the engine, verify fuel pressure on the failing bank, and read VANOS/camshaft adaptation values bank-specifically. If bank 2 cam timing shows a 4+ degree offset from commanded, the bank-2 VANOS solenoid or cam phaser is the cause and cylinder 12 is just the most-affected cylinder in the firing event. Caveat: P0312 with a coolant smell and a slightly low coolant level points to a cracked head or head gasket weeping into cylinder 12 — pressure-test the cooling system before authorizing any mechanical work because a head replacement on a V12 is a 25-hour job.

Vehicle-specific patterns: BMW N74 V12 (760Li F02/G12, Rolls-Royce Ghost/Wraith/Phantom VIII, 2009+) — cylinder 12 misfire frequently traces to the right-bank high-pressure fuel pump; the N74 uses two HPFPs (one per bank). Replace pump and prime per ISTA procedure ($1,400-2,200 in parts). BMW N73 V12 (E65 760i, 2003-2008) — bank-2 VANOS solenoid wear causes cylinder 12 cold-start misfire that clears after warm-up. Mercedes M279 V12 biturbo (S65 AMG, S600 W222, 2014+) — cylinder 12 misfire under load is almost always the rear-bank ignition coil failing from heat soak between the turbo and the firewall. Ferrari 612/599/F12 V12 (F133/F140 platforms) — cylinder 12 spark plug fouling from oil ingestion past tired valve stem seals on cars above 50K miles. Estimated repair: $180 to $4,500.