P1374

P1374 is GM's 24X Reference Signal High to Low fault — the early LS platforms use a 24-tooth crankshaft reluctor wheel pressed onto the crank that generates a 24X reference signal to the ignition control module (or directly to the PCM on later years). The PCM expects the 24X transitions to track in lockstep with the 1X cam reference signal, and when the 24X signal shows an unexpected high-to-low transition (a missing or extra pulse, or noise pickup confusing the edge detector), it sets P1374. On the early LS engines (1999-2002 LS1, LS6, 4.8/5.3/6.0 Vortec) this code shows up most often after a crankshaft replacement, a harmonic balancer swap, or any service that removed the front cover — because the reluctor wheel's relative position to the cam-position sensor needs a PCM relearn (CASE — Crankshaft Angle Sensor Error — relearn) before the PCM trusts the signal. Cheapest-first ladder: (1) If P1374 appeared after recent front-of-engine work, perform the CASE/CKP variation relearn with a Tech 2 or compatible scan tool; many GM-aware tools support it. (2) Inspect the crank sensor connector at the back of the block (early LS) or at the lower front of the timing cover (later LS) for oil contamination and corroded pins. (3) Check the air gap on the crank sensor — it should be a tight fit, no shim required on most LS sensors, but a sensor that backed out 1mm because the bolt loosened will throw P1374. (4) Wiggle-test the CKP harness back to the PCM. (5) Replace the crank sensor ($30-$60 AC Delco) only after the relearn and inspection are done. Don't replace the crank sensor before doing a CKP variation relearn — on early LS PCMs the most common P1374 cause is a missed relearn after a balancer or crank service, and a new sensor won't clear the code without it.

Vehicle-specific patterns: 1999-2002 Camaro / Firebird / Corvette with the LS1/LS6 — early LS PCMs are picky about CKP relearn after any front-cover service, and the reluctor wheel itself can crack or shift on extremely high-mileage examples (200k+) creating a real signal disruption rather than just a relearn fault. 1999-2007 Silverado / Tahoe / Sierra / Yukon with the LS-based Vortec 4.8/5.3/6.0 throws P1374 most commonly after harmonic balancer replacement (the balancer doesn't drive the reluctor but the timing-cover removal often disturbs the sensor mount) or after a flexplate swap where the sensor bracket gets bumped. Some early LS truck VINs were affected by a reluctor-wheel manufacturing tolerance issue — GM addressed certain VIN ranges with reluctor wheel replacement under warranty in the early 2000s. The 2003+ truck LS engines (LM7, LR4, LQ4 later builds) moved the CKP sensor to a more accessible location and the P1374 frequency dropped sharply. Estimated repair: $0 (CASE relearn on a customer's truck after their own balancer install) to $480 (crank sensor + reluctor wheel + relearn at an indy shop).