A refined performance sedan with exotic styling and impressive straight-six technology. The M256 engine with 48V EQ Boost mild hybrid delivers smooth power and efficiency, but the 48V battery system is known for premature failures costing €3,000-4,000 when out of warranty. Air suspension leaks are common on higher-mileage examples. The 9G-TRONIC transmission can exhibit jerky shifting that often requires software updates. Carbon buildup on intake valves is inevitable due to direct injection. Most reliable when maintained meticulously with dealer service history. Verify all recalls completed, especially the critical 48V ground cable fire risk recall.
The 48V mild hybrid system powers the integrated starter-generator (ISG), electric auxiliary compressor (eBooster), and start-stop function. When the battery fails, symptoms include "48V Battery Fault" warnings, hybrid system errors, disabled start-stop, vehicle stalling, weak acceleration, and noisy fans. This is a very common occurrence on 2019-2022 models. Mercedes quotes approximately €2,850 for the battery plus 6-7 hours labor at dealer rates, totaling close to €4,000. Importantly, the 48V battery is NOT covered by extended warranty. Some specialists can repair rather than replace. A class action lawsuit has been filed in the US regarding sudden battery failures. The problem often occurs when vehicles sit for days or weeks. Driving regularly and connecting a 12V battery charger when parked for 3-4 days helps prevent issues.
AIRMATIC air suspension leaks€1,200 - €5,000
Air struts develop leaks from brittle rubber, compressor fails over time · more· less
Air suspension failures are among the most frequently reported problems on CLS AMG models, especially as vehicles age beyond 80,000 km. The rubber around struts becomes brittle, cracked, or punctured. Replacing a single AIRMATIC strut at a dealership ranges from €1,200-2,500 per corner. On average, struts last 80,000-100,000 km depending on driving conditions. It's recommended to replace both front or both rear struts together for balance and performance. Air compressor failure adds €1,100-1,200. Addressing multiple leaks and a failing compressor can total €3,000-5,000. Independent workshops using aftermarket parts (Arnott Industries, for example) offer significantly lower pricing, sometimes half the dealer cost. Symptoms include sagging suspension on one corner, rough ride, or "AIRMATIC Visit Workshop" warnings.
Transmission exhibits harsh shifts, delays, or slipping, especially when cold · more· less
The 9-speed 9G-TRONIC transmission can experience rough shifting (especially neutral to first, or first to second), shifting delays, harsh downshifts that feel like jolts or clunks, and slipping gears. This tends to happen both when cold and warm. Many owners report the transmission being "jerky especially on downshifts and generally confused." One owner reported their GLE43's 9G "kicked so bad when downshifting that the car turned itself off in the middle of the street." Root causes include slight valve body miscalibration, sticky solenoids, low fluid pressure, or software glitches. Mercedes issued a TSB recommending software updates as the first fix. Software updates at the dealer cost €150-300. If the issue persists, valve body replacement or transmission rebuild can cost €2,000-3,500. The 9G-TRONIC requires fluid replacements every 4-5 years depending on mileage. Driving in Comfort or Eco mode in traffic helps reduce harshness.
Carbon buildup on intake valves€800 - €1,500
Direct injection causes inevitable carbon deposits, reducing power and efficiency · more· less
Like all modern direct-injection engines, the M256 is prone to carbon buildup on intake valves, especially with city driving and poor quality fuel. Because fuel is injected directly into the combustion chamber rather than through the intake valves (as in port injection), the valves don't get "washed" by fuel. Consequences include detonation, reduced power, rough idle, and poor fuel economy. The most effective solution is walnut blasting, where the intake manifold is removed and crushed walnut shells are blasted at high pressure to clean the valves. Expect €800-1,500 depending on location and shop. This service is typically needed every 60,000-100,000 km. Using premium fuel and occasional "Italian tune-up" highway runs helps slow carbon accumulation but doesn't prevent it entirely. This is a maintenance item, not a defect, but budgeting for it is essential.
Excessive oil consumption€50 - €400
M256 engine can consume up to 0.8L per 1,000 km, requiring frequent top-ups · more· less
Many M256 owners report excessive oil consumption leading to frequent top-ups. Mercedes states that "depending on driving style, the vehicle consumes up to 0.9 US qt (0.8 liters) of oil over 600 miles (1,000 km)" and notes consumption may be higher when new or with frequent high-speed driving. Modern Mercedes engineering includes lighter pistons with shorter skirts, higher compression, and turbocharging which result in better performance at the expense of increased oil consumption. The M256 uses reduced piston rings and oil cooling ducts which may contribute. This can result in engine wear and potential damage if not addressed promptly. Check oil level every 1,000 km or every other fill-up. Cost is primarily for replacement oil (€50-100 per liter for MB-approved 229.71 spec oil). If consumption becomes severe (>1L per 1,000 km consistently), engine wear or turbo issues should be investigated.
Coolant pump failure€600 - €1,200
Electric water pump integrated with thermostat housing fails around 80,000-120,000 km · more· less
The M256 uses an electric water pump integrated with the thermostat housing. Failures typically occur between 80,000-120,000 km. Symptoms include coolant warning light, engine overheating, or poor cabin heating. The integrated electric design means the part costs €400-600. Labor adds another €200-600 depending on shop rates. This is a known M256 issue alongside timing chain noise and eBooster faults. Ignoring coolant pump failure can lead to catastrophic engine damage from overheating. Check coolant level and condition regularly - low coolant or brown/milky appearance indicates pump or head gasket issues.
While less common than on older Mercedes engines, the M256 can develop timing chain noise. The tensioner has a release valve that weakens over time, allowing oil pressure to bleed off overnight. This results in a distinctive rattle for a few seconds on cold start. If rattle persists beyond a few seconds, the chain, guides, tensioner, and sprockets may all need replacing. Typical repair costs €1,500-3,000 including labor. This is one of the known M256 issues reported by specialists. The M256 uses a timing chain (not belt), so there's no scheduled replacement interval, but chain stretch and tensioner wear are concerns. Listen carefully during cold starts when inspecting any M256-powered vehicle.
Turbocharger wastegate failure€1,500 - €3,500
Twin-scroll turbo develops wastegate or boost control problems over time · more· less
The M256's twin-scroll turbocharger has been known to develop wastegate failures or boost control problems. This can result in reduced power output, increased turbo lag, or boost pressure faults. Turbocharger failures are listed among M256 issues, though less common than 48V battery or air suspension problems. Symptoms include loss of power, unusual whistling or grinding noises, excessive smoke, or check engine light with turbo-related fault codes. Turbo replacement at a dealer can cost €2,500-3,500. Independent specialists may offer rebuild options for €1,500-2,000. Regular oil changes with the correct MB 229.71 specification oil are critical for turbo longevity. Delayed oil changes lead to sludge buildup and reduced turbo lifespan.
Electric auxiliary compressor (eBooster) fault€1,000 - €2,500
48V-powered electric compressor reduces turbo lag but can fail · more· less
The M256 features a BorgWarner electric auxiliary compressor (eBooster) that spins up to 70,000 rpm to reduce turbo lag and improve low-RPM response. It's powered by the 48V EQ Boost system. eBooster faults are mentioned alongside coolant pump failures and timing chain noise as known M256 issues. Neglecting maintenance - especially oil, coolant, and hybrid system care - can lead to eBooster damage. Specific replacement costs are difficult to find but likely range €1,000-2,500 based on the complexity of the 48V system. Symptoms may include reduced low-end power, turbo lag, or fault codes related to boost pressure. The eBooster is part of the sophisticated EQ Boost mild-hybrid system and requires specialized diagnosis.
Sophisticated technology with luxury-level maintenance costs
The CLS 53's M256 engine and 48V EQ Boost system represent cutting-edge technology but come with complexity and potential failure points. The most critical concern is the 48V battery system which fails frequently and costs €3,000-4,000 to replace with no extended warranty coverage. Air suspension and transmission issues are manageable with proper maintenance. Carbon buildup is inevitable and requires periodic cleaning. Overall reliability is acceptable when meticulously maintained with full dealer service history, but repair costs are premium-level. Budget €1,500-2,500 annually for maintenance and another €700-1,400 for unexpected repairs.
Pre-Purchase Inspection Checklist
General checks
Service history
Complete Mercedes-Benz dealer or authorized specialist records essential. Digital Service Booklet history adds resale value. Verify oil changes every 10,000 km or 12 months with MB 229.71 spec oil.
Cold start
Must start engine completely cold. Listen for timing chain rattle in first 10 seconds. Brief rattle (<3 seconds) is normal; longer indicates tensioner issues.
Body condition
Check for accident damage, paint mismatch, panel gaps. CLS bodywork is aluminum and expensive to repair.
Test drive
Minimum 30 minutes including highway speeds to get transmission and 48V system up to temperature.
Specific for this vehicle
Check for 48V battery fault codes
Have dealer scan for fault codes U10F3, P1AFA, U11C3 related to 48V system. Ask owner how often car is driven - sitting for weeks triggers failures. Verify start-stop function works properly.
Test AIRMATIC air suspension operation
Check all four corners for sagging when parked overnight. Listen for compressor running excessively. Look for oil misting on shock bodies. Raise/lower suspension through drive modes to verify function.
Evaluate 9G-TRONIC transmission shifting quality
Drive for at least 20 minutes. Feel for jerky shifts, hesitation, harsh downshifts, or clunking. Test in Comfort, Sport, and Sport+ modes. Cold shifting should be smooth. Check transmission fluid color (should be red, not brown/burnt).
Verify all safety recalls completed
Critical: 48V ground cable fire risk recall (2021-2023 production). Also check 12V battery security, transmission wiring harness reroute, rear seat belt retractor, and DISTRONIC hand detection software. Contact dealer with VIN.
Inspect for oil consumption history
Check if owner has records of oil top-ups between services. More than 1L per 1,000 km indicates excessive consumption. Look for oil residue around turbo or on exhaust manifold.
Check MBUX infotainment for software updates
Verify MBUX isn't glitchy or frozen. Test touchscreen responsiveness, voice control, navigation, and Bluetooth pairing. Ask about Apple CarPlay/Android Auto functionality (early 2019 models lack wireless connectivity).
Test all electronic systems thoroughly
Active Distance Assist DISTRONIC, blind spot assist, lane keeping, automatic emergency braking, 360-degree camera, Burmester sound system, digital cockpit, ambient lighting, massaging seats. Electrical failures are expensive.
Inspect brake wear
CLS 53 has large AMG brakes. Check rotor thickness and pad depth. Rotors should be replaced every 60,000-80,000 km. Front brake rotors wear faster than rear. Replacement costs €500-900 per axle.
Rear seat belt retractor child seat issue (2021 CLS53)Verify completed
DISTRONIC hand detection software error (2021-2022)Software update OTA
9G-TRONIC shift quality TSB (725.1)Software update available
Contact Mercedes-Benz dealer with VIN to verify all recalls completed. The 48V ground cable and transmission wiring harness recalls are fire risks and must be verified. Some recalls were completed via over-the-air (OTA) software updates.
Warranty Status
Factory warranty (4 years / 80,000 km)Expired on all 2018-2021 models
Extended warranty (1-3 year options)€3,000-5,000 depending on coverage
48V battery coverageNOT covered by extended warranty
Extended warranty recommended but 48V battery excluded
All 2018-2021 CLS 53 models are now outside their original 4-year/80,000 km factory warranty. Extended warranty is available through Mercedes-Benz dealers for 1, 2, or 3 additional years with options extending coverage to 75,000 or 100,000 km. However, the 48V battery system - one of the most common and expensive failures - is explicitly NOT covered by extended warranty. Extended warranty does cover engine, transmission, transfer case, drive axle, and most electrical/mechanical components. Consider purchasing extended warranty if available, but budget separately for potential 48V battery failure.
This report is for informational purposes only and does not constitute professional advice. Estimates may be inaccurate. Always have a qualified specialist inspect the vehicle before purchase. We accept no liability for decisions made based on this information.