P2305

P2305 means the PCM detected a fault on the SECONDARY circuit of ignition coil B — the high-voltage side that fires the spark plug, typically diagnosed by the PCM watching burn-time (spark-line duration, healthy is 0.8-2.0 ms) or detecting that the demanded 15-40 kV firing voltage exceeded the coil's capacity. Open-secondary is the most common P2305 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 B and inspect the 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 P2305 cases. (2) Pull the plug and read it — normal-but-dead = no spark reached it; sooty/fouled = secondary fired but plug was contaminated. (3) Ohm the secondary winding through the plug tower — spec is 5K-25K ohms depending on platform; OL confirms an open secondary. (4) Scope the secondary with a kV probe — firing line above 25 kV with a short or missing burn line indicates open secondary, worn plug, or wide gap stressing the coil. Expensive-misdiagnosis caveat: swap coil B to an adjacent cylinder before replacing — if P2305 follows the coil, it's the coil; if it stays, 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 dominates P2305 case files for the same reason it dominates all per-cylinder secondary codes — the long extension boot down into the deep plug well traps moisture, oil mist, and carbon, then carbon-tracks the boot insulation and shorts secondary voltage to the cylinder head before it reaches the plug. Replacing the boot and spring contact at every spark-plug service prevents the majority of these codes. 2005-2014 GM LSx V8 (LS1/LS2/LS3) sees boot rubber hardening and cracking from valve-cover-mounted heat exposure around 80-100K miles. 2011+ Ford 5.0 Coyote uses a shorter boot than the Triton but still tracks around 90K miles. 2007-2018 Toyota 2GR-FE / 2AZ-FE secondary failures tend to be full-coil failures rather than boot-only — Toyota's coil tower is more sealed than the Ford design. Mazda RX-8 Renesis rotary uses paired trailing and leading coils with a much higher firing frequency than a piston engine — leading coils fail first around 50-70K miles, secondary symptoms identical to P2305. Mercedes M271/M273 coil-pack secondary failures cluster at 80-120K miles. Common pattern: coil B usually maps to cylinder 2 or 3 depending on OEM firing-order assignment — verify against the OEM wiring diagram. Estimated repair: $15 (boot + dielectric grease) to $400 (full coil set on a V8).