P0521 on Chevrolet: How to Diagnose & Fix

GM's Active Fuel Management (AFM) system deactivates cylinders to improve fuel economy, but can cause premature lifter wear and failure. Symptoms include rough idle, misfires, and valve train noise. GM released updated lifters.

A subset of 2023 Silverado 1500 trucks (also Colorado and GMC Sierra 1500) built with the 2.7L L3B TurboMax engine left the factory with engine blocks that were cracked or could crack in the main oil gallery, traced to a 'hot core pin' casting defect during block manufacturing. A crack in the main oil gallery — the primary pressurized-oil channel feeding the whole engine — can cause loss of oil pressure and catastrophic engine damage. GM addressed it with Customer Satisfaction Program N232415060 (released February 2024) rather than a formal safety recall; the affected U.S. population was small (reported around a dozen trucks), but the remedy is a full long-block engine replacement with no patch available. Coverage under the program ran through March 31, 2026.

The engine oil pressure sensor/sending unit on LS truck V8s commonly fails or gets its screen clogged with sludge, sending the dash gauge bouncing, pegging high, dropping to zero, or reading falsely low—often with an 'oil pressure' warning even though actual pressure is fine.

The 5.3L V8 with Active Fuel Management (AFM/cylinder deactivation) is prone to two linked failures: excessive oil consumption from carbon deposits in the piston ring grooves caused by oil pulled through the PCV system and oil spray discharged from the AFM pressure relief valve inside the crankcase, and collapsed/failed AFM lifters that can damage the camshaft. Burning oil fouls spark plugs and can trip low-oil shutdown commands; a failed lifter causes a ticking/knocking misfire. GM settled the Siqueiros class action ($150M) over the LC9 5.3L piston-ring defect for 2011-2014 vehicles bought or leased in California, Idaho, or North Carolina.

GM recalled roughly 597,630 trucks and SUVs (NHTSA recall, 2025) — including 2021-2024 Silverado 1500 with the 6.2L L87 V8 — for connecting rods and/or crankshafts that may have manufacturing defects (debris in oil passages or out-of-spec parts). Affected engines can seize or knock and suddenly lose propulsion, with documented fire and crash allegations. GM's tally tied to the recall logged about 28,000 complaints, including ~14,300 alleging loss of propulsion. The remedy splits vehicles into two groups: those that fail inspection get a full engine replacement, while those that pass switch from 0W-20 to thicker 0W-40 oil plus a revised service procedure. A class action is pending, and NHTSA has opened a follow-up engineering analysis into post-recall L87 failures.

GM PIP5947C addresses North American 2024 Trax 1.2L LIH vehicles below 2,000 miles with excess oil consumption, blue exhaust smoke or idle surge. It identifies an internal camshaft-cover failure as the cause.

The Tahoe 5.3L and 6.2L V8 engines with Active Fuel Management (AFM) use collapsible hydraulic lifters that commonly fail between 80,000-120,000 miles. Failed AFM lifters cause misfires on deactivation cylinders (1, 4, 6, 7), ticking noises, and potential camshaft and cylinder wall damage. This is the same issue affecting the Silverado and Escalade. GM issued Service Bulletin 15-06-01-002O for 2015-2020 Tahoes with the 5.3L (L83) or 6.2L (L86), acknowledging collapsed or stuck AFM lifters. GM's bulletin calls for replacing the affected bank of AFM lifters (with new lifter guides), and the Valve Lifter Oil Manifold (VLOM) only when diagnosis finds a fault in it; the camshaft is replaced if lobes or rollers are worn.

Gen V 5.3L engines use variable valve timing (VVT) with a cam phaser and VVT solenoid. Sludge and wear cause the phaser to rattle and the timing chain to stretch, producing a cold-start 'marbles in a can' rattle that often precedes a check-engine light. It sets crank/cam correlation codes (P0008/P0016/P0017) or VVT performance codes (P0011). GM PIP5258A covers P0011 cam actuator codes on the 5.3L L83 that persist after standard diagnosis, directing technicians to check oil pressure and the number 2 cam bearing that feeds the actuator. Ignored, a stretched chain can jump timing and cause severe internal damage.

The C8 Corvette uses the LT2 6.2L V8 with Dynamic Fuel Management (DFM) which can deactivate any combination of cylinders. While the LT2 is generally reliable, some owners report lifter tick similar to the AFM issues on truck engines. The DFM system uses the same collapsible lifter technology. GM bulletin #19-NA-218 (Ticking Noise from the Engine) covers the 2020-2021 LT2 and attributes cold-start tick to a lifter that has not pumped up with oil. The tick is most noticeable on cold starts and usually disappears when warm. Severe cases may require lifter replacement.

The 2.4L Ecotec (LAF/LEA) four-cylinder used in 2010-2017 Equinox (and its GMC Terrain twin) consumes oil at an abnormal rate — frequently as much as 1 quart per 1,000-2,000 miles — due to low-tension/stuck oil-control piston rings combined with PCV-system issues that draw oil past the rings. Owners report blue exhaust smoke on startup and acceleration, low-oil warnings between changes, and in severe cases engine damage when the oil level drops unnoticed. GM acknowledged the defect in a 2015 service bulletin instructing dealers to replace the four piston assemblies on affected engines, and nearly 400 NHTSA complaints describe the same failure. A $42M class-action settlement (final approval Nov 2019) covered 2010-2013 vehicles, and a January 2023 suit extended claims to 2014-2017 units.