The 2020 Silverado 2500HD is a capable heavy-duty truck, but the L8T gas engine has documented catastrophic piston-skirt failures, while the Duramax L5P is significantly more reliable. Transmission oil cooler leaks are a known weak point across both powertrains.
L8T 6.6L Gas Engine Piston Skirt Failure (Catastrophic)
Occasional · high severityTypical onset: 30,000-80,000 mi
Symptoms: heavy metallic knocking especially when cold, loss of power under load, metal shavings in oil, sudden engine seizure in severe cases
Fix: Complete engine rebuild or short-block replacement required. Pistons crack at the skirt, contaminating oil with aluminum debris. TSB covers some cases under powertrain warranty extension, but out-of-pocket requires 25-35 labor hours for R&R plus machine work.
Estimated cost: $8,000-15,000
Allison 10-Speed Transmission Oil Cooler Line Leak
Common · medium severityTypical onset: 50,000-90,000 mi
Symptoms: transmission fluid puddle under driver side, fluid level dropping on dipstick, burnt smell if fluid contacts exhaust, transmission temperature warning in severe cases
Fix: Cooler lines corrode where they connect at radiator and transmission. Requires replacing both metal lines and refreshing fluid. 3-4 labor hours including proper fill/bleed procedure.
Estimated cost: $600-1,200
Transmission Mount Failure (10-Speed)
Occasional · medium severityTypical onset: 60,000-100,000 mi
Symptoms: harsh clunk when shifting into Drive or Reverse, vibration at idle in gear, visible sagging of transmission tailhousing, driveline shudder during acceleration
Fix: Rubber isolator in rear transmission mount deteriorates from heat and load cycles, especially in trucks used for towing. Requires transmission support and 2-3 hours for proper replacement.
Estimated cost: $400-700
Duramax L5P Fuel Filter Housing Air Intrusion
Occasional · medium severitySymptoms: hard starting after sitting overnight, rough idle for 30-60 seconds after cold start, intermittent loss of power, white smoke on startup
Fix: Factory fuel filter head O-rings allow air into system. Replace filter housing assembly or upgrade to aftermarket aluminum head with better seals. 1.5-2 hours labor including prime/bleed.
Estimated cost: $300-600
DEF System Heater Failure (Duramax)
Occasional · medium severityTypical onset: 70,000-110,000 mi
Symptoms: 'DEF system fault' message in cold weather, reduced engine power mode below 20°F, DEF quality warning despite fresh fluid, countdown to speed limitation
Fix: DEF tank heater element fails, preventing proper fluid temperature for injection. Requires dropping DEF tank and replacing heater assembly. 4-5 hours labor in cold-weather states where this is common.
Estimated cost: $800-1,400
FICM (Fuel Injection Control Module) Connector Corrosion - L5P
Rare · high severitySymptoms: intermittent no-start with no crank, random misfires across multiple cylinders, P0087 fuel rail pressure codes, complete engine shutdown while driving
Fix: FICM connector under valve cover exposed to moisture develops corrosion on pins. Requires valve cover removal, connector replacement, and pin cleaning. 3-4 hours labor, worse if corrosion has damaged module.
Estimated cost: $500-2,000
Buy the Duramax version without hesitation if maintenance history is clean; avoid the 2020 L8T gas engine entirely unless it's already had the shortblock replaced under warranty.
AI-assisted summary drawn from NHTSA recall data, our labor-times database, and platform knowledge. Not a substitute for a pre-purchase inspection on a specific vehicle.