The 2021 F-350 Super Duty is built on Ford's proven platform with two robust engine choices, but the 6.7L Power Stroke diesel has developed significant warranty-claim issues around pistons and cooling systems, while the 7.3L Godzilla gas engine proves more reliable though thirsty.
6.7L Power Stroke Piston Failure (Wrist Pin / Skirt Cracking)
Occasional · high severityTypical onset: 30,000-80,000 mi
Symptoms: Sudden onset knocking or slapping noise from bottom end, Metal shavings in oil filter, Loss of power, Check engine light with misfire codes, Catastrophic failure can occur without warning
Fix: Complete engine teardown required, typically replacing all pistons, wrist pins, and inspecting/replacing connecting rods and bearings. Some cases require full short block. 25-35 labor hours depending on cab-on or cab-off approach. Ford extended warranty coverage through 2023 for some VINs.
Estimated cost: $15,000-25,000
10R140 Transmission Oil Cooler Line Failure
Common · medium severityTypical onset: 40,000-90,000 mi
Symptoms: Transmission fluid weeping or spraying from cooler lines at radiator, Low trans fluid warning, Burnt transmission fluid smell, Slipping or delayed engagement if fluid level drops significantly
Fix: Replace cooler lines and fittings at radiator, flush contaminated coolant if cross-contamination occurred. Common on trucks with frequent towing. 3-5 labor hours including system flush and refill.
Estimated cost: $800-1,500
Transmission Mount Failure (Rear Mount)
Common · medium severityTypical onset: 50,000-100,000 mi
Symptoms: Clunking when shifting from park to drive or reverse, Vibration through cab at idle in gear, Excessive drivetrain movement visible under throttle application, Binding sensation during shifts
Fix: Replace rear transmission mount (crossmember mount), inspect front support. Especially common on diesel models with high torque and towing duty. 2-3 labor hours.
Estimated cost: $400-700
6.7L Power Stroke Fuel System Contamination / Filter Clogging
Occasional · medium severitySymptoms: Hard starting when cold, Loss of power under load, Rough idle, Extended cranking, P0093 or P0088 fuel pressure codes
Fix: Premature fuel filter clogging traced to fuel tank debris and poor fuel quality. Requires fuel filter replacement, sometimes fuel tank drop and cleaning if debris source identified. Some cases need injector cleaning or replacement. 2-8 hours depending on scope.
Estimated cost: $300-3,000
6.7L Power Stroke EGR Cooler Clogging (Carryover Issue)
Occasional · medium severityTypical onset: 80,000-150,000 mi
Symptoms: Reduced power, High exhaust gas temperatures, P0401 or P0402 EGR flow codes, White smoke on cold start, Coolant loss without external leaks
Fix: EGR cooler becomes clogged with soot or develops internal coolant leaks. Requires EGR cooler replacement and system cleaning. 8-12 labor hours cab-on, less if cab pulled. More common with short-trip duty cycles.
Estimated cost: $2,500-4,500
7.3L Godzilla Spark Plug Blowout / Thread Damage
Rare · medium severityTypical onset: 60,000-120,000 mi
Symptoms: Sudden misfire on one cylinder, Hissing or ticking noise from cylinder head, Loss of compression on affected cylinder, Check engine light with misfire code
Fix: Aluminum head spark plug threads can strip or allow plug ejection, similar to older Ford modular engines but less frequent. Requires HeliCoil or TimeSert thread repair. 2-4 hours per cylinder.
Estimated cost: $500-900
Buy the 7.3L gas if you can live with the fuel consumption; avoid 2020-2021 6.7L diesels unless comprehensive engine records prove piston recall completion—these are $60K+ trucks that can need $20K engines before 100K miles.
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.