Mercedes-Benz GLC 300 X254

The GLC 300 X254 uses a 48V mild hybrid system with an integrated starter generator (ISG) built into the 9G-Tronic transmission. NHTSA opened an investigation in 2023 after complaints that the 48V system causes frequent engine warning light illumination and no-start conditions. One owner reported a complete shutdown while driving that could not be resolved for over 90 minutes. The root cause involves the 48V system's ground connection, which can come loose, and interactions between the 48V and 12V batteries where the 12V battery drains the 48V system. Mercedes states the car needs to be driven at least 30 km daily to maintain charge. Repair costs depend on the specific component: 48V battery replacement runs approximately €800-1,500, while ISG unit replacement (which requires partial transmission disassembly) can reach €2,500-3,500 at a dealer.

The X254 GLC uses the latest MBUX infotainment system, which has documented reliability issues across multiple Mercedes platforms. Common symptoms include a completely black screen at startup, freezing during use, glitching graphics, and loss of connected functions like Blind Spot Assist. Many cases are resolved by software updates (free under warranty), but some require hardware replacement. A new MBUX display unit costs approximately €1,500-2,500 including coding to the vehicle's VIN, which must be done by a dealer or specialist with official diagnostic tools. Software-only fixes typically cost €100-500 at an independent workshop.

Mercedes UK has acknowledged that 12V battery drain in the GLC X254 is caused by faulty door control modules. The modules draw current when they should be in sleep mode, draining the 12V battery overnight or within a few days of parking. Symptoms include the car not starting, warning messages about low battery, and intermittent door lock malfunctions. The fix requires replacing the affected door modules, though there have been reports of a 3-4 month wait for parts in some markets. Each door module costs approximately €200-300 including labour. Some owners have had success with a 16-module software update that resolved the parasitic drain without hardware replacement.

Like all modern direct-injection engines, the M254 does not wash fuel over the intake valves, allowing carbon deposits to accumulate. This typically becomes noticeable between 50,000-80,000 km with symptoms including rough idle, hesitation under acceleration, and reduced fuel economy. Walnut blasting is the standard cleaning method, costing approximately €500-1,000 depending on the shop. Some independent specialists offer this for €400-600. This is a recurring issue that may need to be repeated every 50,000-60,000 km, especially with urban driving patterns.

Some GLC 300 owners report a noticeable jolt or jerk during low-speed shifts, typically between 3rd and 2nd gear when coming to a stop. The issue appears to affect both the X253 and X254 generations. In most cases, a software update or transmission adaptation reset resolves the problem at minimal cost (€100-300). In rare cases where the valve body requires replacement, costs can reach €1,500-2,000. The 9G-Tronic is generally considered a robust transmission, and mechanical failures are uncommon compared to software calibration issues.

Mercedes-Benz specifies that up to 0.5 litres per 1,000 km is within acceptable limits for the M254 engine. However, some owners of early production models (2021-2023 build dates) have reported consumption rates at or above this threshold. The issue is more prevalent in engines that are driven aggressively or in stop-and-go traffic. Mercedes has not issued a formal recall but some dealers have replaced engines under goodwill for loyal customers with complete service histories. If oil consumption is excessive, the typical repair involves piston ring replacement (€1,000-1,500 at an independent specialist) or engine replacement under warranty/goodwill.