2000 GMC P1351 Code Fix: Original Key Solves Intermittent Starting Issue (No Chip Required)

I have a 2000 GMC Quad Cab 4WD with a Vortec 350 engine. This truck originally did not have a key chip, which led me to believe the issue wasn't related to immobilizer systems. Recently, I started receiving a P1351 diagnostic code — indicating an issue with the ignition system security. The truck would start intermittently and often fail to start reliably, even after replacing numerous components including the coil, battery, ignition control module, distributor, spark plugs, wires, starter, voltage regulator/alternator, and crankshaft position sensor. I checked for wiring shorts and repaired frayed sections, but each repair only temporarily resolved the issue. The 'Service Engine Soon' (SES) light would go off after a few starts or repairs, then return within 10–20 attempts. After consulting with an experienced mechanic who suggested trying an original key, I tested it. Using my original key — which was worn from over 15 years of daily use — the SES light turned off immediately and the truck started reliably. I later had a new key made at Home Depot using the original key as a template. During the day, I drove the vehicle and intentionally attempted to start it 30 times. The P1351 code did not return, and all operations were stable. The mechanic explained that even without a chip in the key, older vehicles like this one use magnetic tumbler mechanisms in the ignition lock cylinder as part of their security system. A worn key may not engage the tumblers properly, leading to inconsistent activation of the start-up security system — which prevents proper engine cranking. This explains why the truck would start sometimes and fail at others, even with all mechanical components replaced. Important note: The original key can be pulled out of the ignition while the engine is running (a feature I appreciated for farm use), but the new key does not allow this. So far, everything works perfectly — the mystery is solved. This experience confirms that key condition and tumbler engagement play a critical role in vehicles with P1351 codes, even without chip-based security.