P1351

P1351 is set when the PCM detects an unexpected high voltage on the ignition control (IC) circuit for one or more coils — typically the bypass or EST line on distributor-equipped GM engines, or the individual coil driver feedback line on coil-near-plug platforms. The PCM expects the IC line to be pulled low when it commands spark and to rise to system voltage during the off interval; a continuous high reading suggests an open driver, a shorted-high signal wire, or a coil module that isn't loading the line correctly. Diagnose cheapest first: with key-on engine-off, backprobe the IC wire at the coil/ICM and verify it sits near 0V (the PCM holds the line low at rest on most platforms); if it reads battery voltage, disconnect the PCM connector and re-check — if it stays high, the wire is shorted to power; if it now reads zero, the PCM driver is suspect. The expensive misdiagnosis: throwing a coil pack or ICM at the truck when the actual fault is a chafed IC wire arcing to a valve cover bolt under hard acceleration.

Vehicle-specific patterns: 1996-2005 S10/Blazer/Sonoma 4.3L Vortec sees P1351 from ICM (ignition control module) failure under the distributor cap — the module's heat-paste dries and the driver fails high. 2000-2005 Cavalier/Sunfire 2.2L Ecotec sets P1351 when the coil-on-plug harness chafes against the cam cover stud at cylinder #1. 1999-2007 Silverado/Sierra 4.8L/5.3L/6.0L gets P1351 alongside multiple cylinder misfires when the underhood fuse block develops an internal solder crack on the ignition feed bus. 2004-2008 Grand Prix 3.8L Series III throws P1351 from the ICM mounted on the rear coil pack overheating against the firewall. Estimated repair: $80 to $480.