Mercedes-Benz AMG GLC 43
The M139 is the logical development of the M133 and surpasses it in every respect: more power, better thermal management through an improved cooling concept, combined direct and port injection to fight intake valve carbon build-up. As a young engine with limited long-term data it is considered robust so far. Isolated reports of coolant sensor issues and water pump failures at high speeds. Tuning potential is enormous, but fuel supply must be upgraded at higher power levels.
AMG Four-Cylinder Beast
World's most powerful production four-cylinder with 310 kW in the GLC. Drift mode included — absurdly fast.
Engine Weaknesses 6
The world's most powerful series-production four-cylinder at 421 hp subjects its turbocharger to extreme stress. Without adequate cool-down periods after full-load operation, premature bearing wear is likely.
Symptoms: Whistling at high load, oil mist in intake tract, power drop in heat
The high-performance turbocharger on the M139 can push oil into the intake tract due to excessive shaft play. Early vehicles were subject to a recall for turbocharger bearing defects.
Symptoms: Oil consumption without visible leaks, smoke cloud on cold start, power drop under high load
The M139 cooling system is designed at its limits. After extended high-load phases coolant temperature can reach critical values, especially in warm weather.
Symptoms: Coolant temperature warning after spirited driving, restricted engine power through thermal management
Individual owners report coolant temperature warnings at high-speed driving. Diagnosis: faulty coolant pump or sensor. Isolated cases, no systematic problem known.
Symptoms: Coolant temperature warning on display, especially at high speed (170+ km/h)
Despite combined direct and port injection the M139 can build up deposits on intake valves under intensive use. Considerably less than pure DI engines.
Symptoms: Slight hesitation at part throttle, minimal power loss, rarely noticeable
The M139 shows elevated oil consumption under high-load operation. As a high-performance engine with 7.0-litre oil capacity, regular oil level checks are mandatory.
Symptoms: Dropping oil level between service intervals, slight oil smell on hot engine
Vehicle Weaknesses 11
A software fault in the engine management incorrectly detects misfires and cuts the fuel injection. Approximately 60,000 vehicles affected.
Inadequate attachment of the acoustic pad in the B-pillar. The pad can detach and impair the seat belt retractor. Belt could fail in an accident.
Acoustic pads on the B-pillar can detach and jam the seat belt retractor, preventing the pretensioner from deploying. 908 vehicles affected.
Faulty parking sensors and camera systems trigger false warnings. In snowfall, the emergency brake assist activates unintentionally.
The MBUX system freezes or becomes unresponsive. Assistance systems can be permanently deactivated. OTA updates often only temporarily relieve the symptoms.
The C254 also develops noises at the front axle when turning. Ball joints and strut top mounts wear faster with larger wheels.
MBUX system shows sporadic crashes, sluggish responses and connection drops, especially in early production months. Software updates at the workshop frequently needed.
Park assist detects non-existent obstacles, brake assist brakes without reason in snowfall, blind spot monitor fails. False alarms can trigger dangerous situations.
The MBUX system gets very hot on longer journeys. The display shows visual noise or goes black in heat. The fault disappears after cooling down.
Early C254 models show software faults: rear camera stays black, driver assistance fails, screen freezes. Usually resolvable after OTA updates.
Rattling passenger door on cobblestones, creaking centre console above 60 km/h, vibrations in the accelerator pedal. Ambient lighting elements detach.
Reports & Tests
11 owner complaints filed with NHTSA (2022–2025). Most reported: Electrical (8), Collision Avoidance (3), Seats (2).