The 2021 720S is McLaren's mid-generation M840T-powered supercar with known piston/liner issues that can appear surprisingly early. When healthy, it's brilliant; when not, engine rebuilds are eye-wateringly expensive and increasingly common on hard-driven or heat-cycled examples.
Piston Ring / Cylinder Liner Failure (M840T Engine)
Occasional · high severityTypical onset: 15,000-40,000 mi
Symptoms: Excessive oil consumption (1+ quart per 1,000 miles), Blue smoke on startup or under load, Loss of compression, misfires, rough idle, Metal debris in oil during analysis
Fix: Full engine-out rebuild required: replace pistons, rings, machine or re-sleeve liners, new bearings. 60-80 hours labor depending on machine shop turnaround. McLaren extended warranty coverage varies by market and build date.
Estimated cost: $35,000-55,000
Transmission Oil Cooler Leaks
Common · medium severityTypical onset: 20,000-50,000 mi
Symptoms: Transmission fluid drips under center of car, Burnt smell after spirited driving, Transmission overheat warnings on track, Low fluid level warnings
Fix: Replace cooler assembly and hoses; rear bodywork removal required for access. 8-12 hours labor. Some units have revised cooler design from factory; aftermarket upgrades available.
Estimated cost: $3,500-6,000
Transmission Mount Failure
Occasional · medium severityTypical onset: 25,000-60,000 mi
Symptoms: Clunking on hard acceleration or deceleration, Vibration through cabin at idle in gear, Excessive driveline movement visible from below, Grinding noises during shifts
Fix: Replace hydraulic transmission mount. Requires rear subframe support and partial exhaust removal. 6-9 hours labor. OEM part only; no viable aftermarket.
Estimated cost: $2,800-4,200
Shift Solenoid Pack Failure (7-Speed SSG)
Occasional · medium severityTypical onset: 30,000-70,000 mi
Symptoms: Harsh or delayed shifts, especially 2-3 and 5-6, Transmission fault codes, limp mode, Inability to select certain gears, Jerky behavior in automatic mode
Fix: Replace solenoid assembly; requires transmission pan drop and valve body access. 5-7 hours labor. Software calibration update recommended simultaneously.
Estimated cost: $2,200-3,800
High-Pressure Fuel Filter Clogging
Occasional · medium severitySymptoms: Hesitation or stumble at high RPM, Reduced power above 6,000 RPM, Long crank times, especially when hot, Lean fuel trim codes
Fix: Replace high-pressure fuel filter in tank module. Tank drop required, 4-6 hours labor. Often contaminated by poor fuel quality or tank debris. McLaren recommends every 3 years regardless of miles.
Estimated cost: $1,800-2,800
Crankshaft Position Sensor Dropout
Rare · high severitySymptoms: Random no-start condition, crank but no fire, Stalling while driving with immediate restart ability, Intermittent loss of tachometer signal, Engine cuts out momentarily under load
Fix: Replace crankshaft position sensor; requires access from underneath, 2-3 hours labor. Confirm wiring harness integrity before replacing sensor. Can strand you unexpectedly.
Estimated cost: $800-1,400
Spectacular performance and engineering, but the M840T piston lottery is real—buy only with comprehensive warranty or deep reserves for potential engine rebuild; not a casual ownership proposition.
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.