Mercedes M256 Engine: A Modern Straight-Six With a Few Growing Pains

For decades. Mercedes-Benz was synonymous with the smooth, refined inline-six. Then came nearly 20 years of V6s. But in 2017, the German automaker brought the straight-six back with the M256, a high-tech powerhouse featuring a 48-volt mild hybrid system, an electric auxiliary compressor, and a twin-scroll turbo.

It’s now under the hood of everything from the S-Class to the G-Wagon. But as with any relatively new engine, real-world ownership has revealed a handful of hiccups.

The biggest headaches so far center on the M256’s 48-volt electrical system. A 2024 recall (NHTSA 24V-207) flagged a loose ground connection under the front passenger seat in GLE and GLS models, a risk of fire due to electrical resistance. Mercedes also issued a 2025 technical service bulletin for 2019-2021 CLS, E-Class, GLE, GLS, and AMG GT models where the 48V battery control unit software didn’t match specs, potentially masking a check-engine light.

Both fixes were warranty-covered. Then there’s the direct-injection carbon buildup on intake valves, a common issue since fuel never washes those valves. And a 2019 recall (19V-605) hit 3,264 CLS models for an electric coolant pump wire that could chafe against the engine fan, risking overheating or a 48V shutdown.

Some owners also report a cold-start rattle that suggests timing chain wear, but it’s too early to call that a pattern.

Despite those issues, the M256 has earned widespread praise for its silky power delivery. Hagerty called it “sublimely smooth” in the S500. And Mercedes is leaning into it: the brand recently confirmed it’s dropping the turbocharged four-cylinder C63 in favor of a six-cylinder C53 using this same M256 architecture.

” The engine now spans 362 to 443 horsepower across the lineup. Critics note it lacks the drama of a V8, but for daily driving, it’s a polished, capable return to form, just keep an eye on those electrical gremlins and coolant system quirks.