P2308

P2308 means the PCM detected a fault on the SECONDARY circuit of ignition coil C — the high-voltage side that fires the spark plug, diagnosed by the PCM watching burn-time (0.8-2.0 ms for a healthy coil firing a fresh plug) or detecting that the demanded 15-40 kV firing voltage exceeded the coil's capacity. Open-secondary is the most common subcondition: failed secondary winding, carbon-tracked boot, or burned-through spring contact between the coil tower and the plug terminal. Cheapest-first ladder: (1) Pull coil C and inspect the spark-plug boot for carbon tracks (wandering black lines on the inside insulation), oil contamination from a leaking valve-cover gasket, or arc-through holes — a $15 boot + dielectric grease resolves a large share of secondary-circuit codes without touching the coil. (2) Pull the plug and read it — normal-but-dead = no spark reached it; sooty/fouled = secondary fired but plug contamination. (3) Ohm the secondary winding through the plug tower — 5K-25K ohms is normal; OL confirms open secondary. (4) Scope the secondary with a kV probe during snap-throttle — firing line above 25 kV with a short or missing burn line indicates open secondary, worn plug, or excessive gap. Expensive-misdiagnosis caveat: swap coil C to an adjacent cylinder before replacing — if P2308 follows the coil to the new cylinder, it's the coil; if it stays on the original cylinder, the spark plug, plug well, or harness is the cause. A $40 coil swap-test saves $80.

Vehicle-specific patterns: 2004-2010 Ford 4.6L/5.4L 3V Triton (F-150, Crown Vic, Expedition) is the most common P2308 platform in North American shops — the long extension boot down into the deep plug well traps moisture, oil mist from leaking valve-cover gaskets, and carbon residue, all of which carbon-track the boot and short the secondary to the cylinder head. Replace the boot and spring contact at every spark-plug service to prevent it. 2005-2014 GM LSx V8 (LS1/LS2/LS3 in Silverado/Camaro/Corvette) sees boot rubber hardening from valve-cover-mounted heat exposure around 80-100K miles. 2011+ Ford 5.0 Coyote uses a shorter boot than the Triton but still sees boot-tracking around 90K miles. 2007-2018 Toyota 2GR-FE 3.5L and 2AZ-FE 2.4L (Camry, Highlander, Sienna, RAV4) secondary failures tend to be full-coil failures rather than boot-only issues — Toyota coils typically last 100-150K miles. Mazda RX-8 Renesis rotary uses paired trailing/leading coils with much higher firing frequency than a piston engine — leading coils fail first around 50-70K miles. Mercedes M271 (1.8L supercharged) and M273 (5.5L V8) coil secondary failures cluster at 80-120K miles. Common pattern: coil C usually = cylinder 3 or 5 on most platforms but verify against the OEM wiring diagram before swap-testing. Estimated repair: $15 (boot + dielectric grease) to $400 (full coil set on a V8).