2021 Mercedes-Benz C-Class Reliability Report

Engine and transmission failures that strand the car make up most of these reports. Cylinder head failures with misfires, rough running and overheating appear from about 42,000 to 81,000 miles, and in one case the replacement engine seized during the dealer's own test drive. Some transmissions fail outright at 15,000 to 22,000 miles. Other reports describe limp mode, warnings that the car cannot shift, total shutdowns at 55 mph with no restart, and 48-volt system faults that cut all propulsion.

Sudden, unwanted slowing with adaptive cruise engaged is the main pattern. The system misreads road signs, including exit-ramp and minimum-speed signs, and cuts speed abruptly, such as from 75 to 45 mph or 65 to 20 mph on the highway. One such event preceded a crash with an injury. Dealer software updates often fail to stop it, and owners find relief only after turning off speed-limit adoption features. Separate reports describe hesitation, reduced-power mode, and stalling at stops when the engine shuts off to save fuel.

Brakes that fail to stop the car, or apply on their own, are the core problem. Two crashes follow low-speed brake failures while parking or creeping, with passengers seeking medical care. Other owners describe the car braking hard in light traffic with nothing ahead, brakes locking up, a parking brake that seizes and immobilizes the car, and an idling car nudging forward in Park. Warning lights rarely appear beforehand, and dealers often cannot reproduce the fault.

Fast wear and tire damage are the main complaints. Original Continental and Cooper tires wear near bald by about 22,000 to 26,500 miles, and owners describe poor traction in the wet. Cars on 21-inch wheels suffer repeated blowouts and sidewall bubbles after potholes, with no spare tire provided. Other reports describe tread separation causing vibration and flapping at 70 mph, and shaking with loud tire noise on wet pavement. One crash into a pole is filed under this category.

Front ball joint separation is the clearest pattern. The right front ball joint breaks away from the lower control arm at low speed, the wheel collapses, and the car cannot move. One case ends in a collision with a parked car. Other reports describe skidding on snow and ice on summer tires, debris striking the underside before the airbags deployed, and a new car delivered with seat, steering and warning-light faults. One crash report lists 15 injuries but contains no usable details.

Warning lights and injuries from deployment make up these reports. In one crash, the steering wheel air bag splits open and lower air bags leave the driver with cuts and burns. Other owners see passenger air bag malfunction warnings, sometimes alongside a burning odor, abnormal noises, or blind spot and active brake assist faults. Dealers either cannot duplicate the problem or fail to fix it after as many as five visits.

Belts that fail to lock or latch are the common thread. One report says the driver's belt did not restrain the driver in a high-speed rear-end collision, leading to a head injury. Another describes a driver's-side retractor that does not lock. In a third case, a third-row buckle latch fails during hard braking and children fall from their seats; the dealer replaces the latch. None of the driver's-side failures has been reproduced by a dealer.