2021 GMC SIERRA 2500HD

Typical onset: 30,000-80,000 mi

Symptoms: metallic knocking/tapping from engine bay especially under load, sudden loss of oil pressure, metal shavings in oil during change, check engine light with low oil pressure codes, complete engine seizure in worst cases

Fix: L8T has inadequate piston oil-squirter design causing oil starvation to rod bearings under high load/heat. Fix requires complete engine rebuild or factory short-block replacement. We're seeing 25-35 hours labor for short block swap including R&R, new gaskets, fluids, timing components. Some cases need full long-block.

Estimated cost: $15,000-25,000

Typical onset: 40,000-90,000 mi

Symptoms: transmission fluid leaking near radiator, pink fluid on driveway, burnt transmission fluid smell, transmission overheating warning, slipping gears if fluid level drops significantly

Fix: Factory cooler lines fail at crimp points or where they connect to radiator. Not just lines—sometimes need radiator-mounted cooler itself if internal leak contaminates coolant. 3-5 hours labor for lines, 6-8 if radiator cooler is compromised and needs flush of both systems.

Estimated cost: $600-1,800

Typical onset: 20,000-60,000 mi

Symptoms: hard or delayed 2-3 and 3-4 shifts especially when cold, shuddering during light acceleration 25-45 mph, clunking into gear from park, transmission adapts poorly after battery disconnect

Fix: Often related to torque converter lockup clutch or valve body issues in the 10L1000 Allison. TSBs exist for software recalibration (2 hours with programming), but many need valve body replacement or torque converter. Fluid flush with OE Dex-VI sometimes temporarily helps. Valve body job is 8-12 hours.

Estimated cost: $500-4,500

Typical onset: 50,000-100,000 mi

Symptoms: DEF system fault messages, reduced engine power mode, countdown to 55 mph derate, DEF fluid weeping from tank or injector, check engine light with P20EE or P207F codes

Fix: DEF tank heater, level sensor, or injector failures. Cold-climate trucks see more issues. DEF injector replacement is 3-4 hours, tank/sender unit is 4-6 hours. Must use OE parts—aftermarket causes more grief. Sometimes lines crystallize and need full system flush.

Estimated cost: $800-2,800

Typical onset: 60,000-120,000 mi

Symptoms: hard starting especially after sitting overnight, extended cranking when cold, loss of prime requiring repriming, fuel weeping from filter housing, rough idle or stuttering acceleration

Fix: Factory fuel filter housing develops cracks or o-ring leaks allowing air into system. Replace entire housing assembly with updated GM part or aftermarket billet housing. 2-3 hours labor including bleeding system. Preventive replacement recommended at 60k miles if towing heavy regularly.

Estimated cost: $400-900

Typical onset: 40,000-80,000 mi

Symptoms: clunk when shifting into drive or reverse, excessive driveline vibration especially under acceleration, visible sagging of transmission tailhousing, knocking from under cab over bumps

Fix: Transmission mount rubber deteriorates faster on heavy-duty use, particularly with plows or heavy trailers. Common on both gas and diesel. Replacement is straightforward: support transmission, swap mount. 1.5-2 hours labor. Inspect during every transmission service.

Estimated cost: $250-450