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.

Certain 6.2L L87 V8 engines left the factory with manufacturing defects in the connecting rods and/or crankshaft — sediment/debris left in the crankshaft oil galleries and out-of-spec crankshaft finish starve the rod bearings of oil. The bearings spin, the rod can knock or seize, and the engine can suffer a sudden loss of propulsion at speed. This is the subject of a large 2025 safety recall covering roughly 600,000 U.S. trucks and SUVs (Silverado 1500, Tahoe, Suburban, Sierra 1500, Yukon, Escalade).

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 runs 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. GM added a screen around the sensor mid-2007 that made the debris problem worse. A leading, cheap-to-fix false-alarm complaint that scares owners into thinking the pump failed.

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 has released updated VLOM and lifter designs for the LT2. The tick is most noticeable on cold starts and usually disappears when warm. Severe cases may require lifter replacement.

L87 6.2L V8s built March 1, 2021 through May 31, 2024 can suffer sudden rod-bearing failure and engine seizure with loss of propulsion. GM attributes it to supplier manufacturing/quality issues — sediment/debris left on connecting rods and in crankshaft oil galleries plus out-of-spec dimensions — that starve and pound the rod bearings. NHTSA safety recall 25V-274 (April 2025) covers 44,802 Tahoes among ~600k GM full-size trucks/SUVs; GM logged over 14,000 field complaints and a dozen alleged crashes/injuries. NHTSA has since opened follow-up query RQ26001 into failures on engines outside the recall and on some units that failed after the dealer remedy.

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 TSB #18-NA-077 acknowledging the problem. The VLOM (Valve Lifter Oil Manifold) must be replaced along with all 16 lifters.

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 TSB PIP5921B also documents a rattle/tick from the intake cam actuator when its cover-plate screws back out. Ignored, a stretched chain can jump timing and cause severe internal damage.

The 3.6L V6 in the Traverse consumes oil at a rate that many owners find unacceptable, particularly on 2009-2017 models with the LLT and LFX engine variants. Owners report consuming 1 quart every 2,000-3,000 miles. The oil consumption is related to piston ring design and the PCV system routing oil vapor back into the intake. The issue is not accompanied by visible external leaks. GM has acknowledged elevated oil consumption on the 3.6L but considers up to 1 quart per 2,000 miles within their acceptable range.

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. NHTSA opened a follow-up engineering analysis into post-recall L87 failures, and a class action is pending — this is currently the highest-profile, highest-demand Silverado issue.

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.

The 5.3L V8 with Active Fuel Management (AFM/cylinder deactivation) is prone to two linked failures: excessive oil consumption from worn piston rings and an AFM oil deflector that sprays oil at the cylinder walls, 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 built on/after Feb 10, 2011.