P0448

P0448 is the opposite electrical failure from P0447 - the ECM commanded the EVAP vent valve and detected a short circuit on the control side, either the solenoid coil is shorted internally, the control wire is shorted to ground or to 12V, or the PCM ground-side driver has failed shorted. Cheapest-first ladder: locate the vent valve at the canister and unplug the connector. Measure solenoid coil resistance across the two pins - a healthy vent solenoid reads 20 to 80 ohms; a short reads under 5 ohms (the coil is internally fused together) and trips the ECM short-circuit protection. If the solenoid resistance is in spec, key-on backprobe the harness side and verify the control wire is not shorted to chassis ground (should read open with connector unplugged, key-off) or to 12V (should read 0V with connector unplugged, key-on engine off). Inspect the connector and pigtail for chafed insulation where the wires cross sharp metal edges on the frame rail or the canister bracket - chafed-through insulation against bare metal is the dominant root cause on truck platforms. The expensive misdiagnosis is replacing the PCM for a shorted driver when the actual fault is a $4 piece of harness loom and 15 minutes with electrical tape - always isolate the short to the solenoid, the wiring, or the PCM before reaching for a control module.

Vehicle-specific patterns: 2007-2014 GM full-size trucks (Silverado, Sierra, Suburban, Tahoe) throw P0448 from rodent damage that simultaneously shorts the vent valve control wire to ground - mice chew through the insulation and the bare conductor touches the canister bracket. 2009-2014 Ford F-150 has the same rodent-induced short pattern on the frame-rail harness. 2007-2014 Toyota Tundra/Sequoia 5.7L throws P0448 from internally shorted vent solenoid coils after long-term road salt and water intrusion - common on trucks driven on salted northern roads. 2009-2018 RAM 1500 throws P0448 from chafed vent valve wiring against the fuel tank strap. Subaru EJ-series (Outback, Forester, Legacy) sees vent valve coil shorts from heat-aged insulation when the solenoid is mounted near the exhaust manifold. The damage pattern across all rodent-affected platforms is identical: check the canister area for droppings, chewed insulation, and nest material before the trip back to the shop. Estimated repair: $50 to $280.