The 2020 CLS with the M256 3.0L turbo inline-six is generally reliable but suffers from a catastrophic engine defect in certain production runs—bearing failures that destroy the bottom end—plus typical Mercedes aging issues with cooling and mounts. Most examples run fine, but when the engine goes, it's catastrophic.
M256 Crankshaft Bearing Failure (Catastrophic)
Occasional · high severityTypical onset: 20,000-60,000 mi
Symptoms: Metallic knocking from engine bay, especially at idle or low RPM, Oil pressure warning light or low oil pressure readings, Metal shavings in oil during change, Sudden catastrophic engine failure with no warning
Fix: Complete short block replacement or engine rebuild required. 25-35 hours labor depending on whether you pull the engine. Many techs recommend full long-block replacement given contamination risk. This is a known defect in certain production batches—Mercedes issued a service campaign but not a full recall.
Estimated cost: $15,000-25,000
Transmission Oil Cooler Line Leaks
Common · medium severityTypical onset: 50,000-80,000 mi
Symptoms: Transmission fluid spots under vehicle, typically passenger side, Low transmission fluid warning on dash, Burnt smell from engine bay, Harsh or delayed shifts if fluid level drops significantly
Fix: Replace cooler lines and reseal connections. The quick-connect fittings and rubber sections deteriorate from heat cycles. 3-5 hours labor including fluid flush and refill with correct MB 236.17 spec fluid.
Estimated cost: $800-1,400
Transmission Mount Failure
Common · low severityTypical onset: 60,000-90,000 mi
Symptoms: Clunk or thud when shifting from Park to Drive or Reverse, Excessive vibration at idle in gear, Visible drivetrainmovement when rocking vehicle in gear with brake applied, Transmission tunnel vibration during acceleration
Fix: Replace transmission mount assembly. The hydraulic fluid in the mount leaks out and the rubber tears. Straightforward job at 2-3 hours labor. Use OEM—aftermarket mounts fail quickly on this chassis.
Estimated cost: $600-900
High-Pressure Fuel Pump Failure
Occasional · high severityTypical onset: 40,000-80,000 mi
Symptoms: Long crank time before engine starts, especially when hot, Rough idle or misfires under load, Loss of power during acceleration, Check engine light with fuel pressure codes (P0087, P0088), Engine may not start at all when pump fully fails
Fix: Replace high-pressure fuel pump on engine. The cam-driven pump on the M256 can fail internally, starving the direct injection system. 4-6 hours labor—pump is buried under intake components. Must use genuine Mercedes pump.
Estimated cost: $2,200-3,500
Coolant Thermostat Housing Leaks
Occasional · medium severityTypical onset: 70,000-100,000 mi
Symptoms: Coolant smell from engine bay, Visible coolant weeping from front of engine, Slow coolant loss requiring top-ups, Coolant warning light on dash
Fix: Replace thermostat housing assembly and associated seals. Plastic housing develops cracks at mounting points. 3-4 hours labor. Do the coolant flush at the same time—these engines are sensitive to old coolant.
Estimated cost: $700-1,200
Airmatic Suspension Compressor Wear
Occasional · medium severityTypical onset: 80,000-120,000 mi
Symptoms: Suspension warning light on dash, Vehicle sits lower after sitting overnight or after long drives, Compressor runs excessively—can hear it cycling frequently, Rough ride or reduced ground clearance
Fix: Replace Airmatic compressor assembly. The compressor motor and valve block wear out from constant cycling. Check struts and lines first—sometimes leaks cause excessive compressor wear. 3-4 hours labor if compressor only.
Estimated cost: $2,000-3,200
Beautiful car with strong performance, but the bearing failure issue is a Russian roulette scenario that can cost more than the car's value—only buy with extended warranty or verified bearing campaign completion.
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.