The 2020 GLS450 X167 with the M256 3.0L inline-six is generally solid, but early examples suffer from catastrophic engine failures due to defective piston rings and bearing issues—warranty extensions cover some cases, but out-of-warranty repairs are financially devastating.
M256 Engine Piston Ring and Bearing Failure
Occasional · high severity
Typical onset: 30,000-80,000 mi
Symptoms: Excessive oil consumption (quart every 500-1000 miles), Blue smoke from exhaust on cold start or acceleration, Metal shavings in oil during changes, Low oil pressure warning, lifter tick, or rod knock in severe cases
Fix: Complete engine teardown to replace piston rings, sometimes pistons, and rod/main bearings if damaged. Mercedes issued warranty extension to 15 years/150k miles for eligible VINs, but many early 2020s fall outside coverage. Short block replacement needed in worst cases—60-80 labor hours total including R&I and ancillaries.
Estimated cost: $18,000-35,000
9G-TRONIC Transmission Oil Cooler Line Leaks
Common · medium severity
Typical onset: 40,000-70,000 mi
Symptoms: Transmission fluid puddle under vehicle, typically passenger side, Burnt transmission fluid smell, Low fluid level causing harsh shifts or slipping, Check engine light with transmission temperature codes
Fix: Replace oil cooler lines where they connect to radiator or transmission—rubber hoses crack and metal fittings corrode. Often requires dropping subframe or extensive disassembly for access. 4-6 labor hours plus fluid flush.
Estimated cost: $1,200-2,200
Transmission Mount Collapse
Occasional · medium severity
Typical onset: 50,000-90,000 mi
Symptoms: Clunk or thud on acceleration from stop, Vibration at idle in Drive that disappears in Park/Neutral, Visible sag or tear in rubber portion of mount, Driveline shudder during gear changes
Fix: Replace transmission mount—the rubber separates from metal housing. Requires supporting transmission and subframe work. 3-4 labor hours.
Estimated cost: $800-1,400
High-Pressure Fuel Filter Clogging
Occasional · medium severity
Typical onset: 60,000-100,000 mi
Symptoms: Hesitation or stumble during acceleration, especially uphill, Hard starting when engine is hot, Reduced power, limp mode activation, Fuel pressure fault codes (P0087, P228C)
Fix: Replace high-pressure fuel filter in engine bay—not the in-tank strainer. Direct-injection systems are sensitive to contamination. 2-3 labor hours including fuel system depressurization and priming.
Estimated cost: $600-1,100
AIRMATIC Air Suspension Compressor Overheating
Occasional · medium severity
Typical onset: 70,000-110,000 mi
Symptoms: Vehicle sags at one or more corners after sitting, Compressor runs continuously or cycles excessively, Suspension warning on dash, vehicle lowered to bump stops, Compressor hot to touch, thermal shutdown codes
Fix: Usually caused by leaking air spring or valve block forcing compressor to overwork. Diagnose leak source first—if compressor is damaged, replace unit and address root cause. Compressor replacement is 3-4 hours; if springs are also shot, add 2 hours per corner.
Estimated cost: $2,000-4,500
mbux Infotainment Screen Freezing and Rebooting
Common · low severity
Symptoms: Screen goes black and restarts while driving, Touchscreen unresponsive, requires hard reset, Camera views freeze or show corrupted image, Bluetooth and navigation stop working intermittently
Fix: Software bug in early MBUX systems—Mercedes released multiple updates. Dealer reflash typically resolves, but some units need head unit replacement if hardware fault. 1-2 hours for software update, 4-6 hours if head unit swap required.
Estimated cost: $200-3,500
Owner tips
Check oil level every 500 miles religiously—early warning of ring failure is consumption spike; catching it before bearing damage saves $20k
Use only MB-approved 229.52 spec oil; aftermarket fluids accelerate M256 timing chain and turbo wear
Verify VIN eligibility for Mercedes warranty extension program before purchase—coverage dramatically changes ownership math
Budget $2k/year for air suspension maintenance after 80k miles; it's expensive but non-negotiable on this platform
Buy only with verified warranty extension coverage or comprehensive aftermarket warranty—the M256 engine grenade risk is too expensive to gamble on otherwise, but the chassis and tech are excellent when powertrain is healthy.
AI-assisted summary drawn from NHTSA recall data, our labor-times database, and platform knowledge. Not a substitute for a pre-purchase inspection on a specific vehicle.
Fitment notes: AGM required for start-stop system; battery located under hood on right side
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Every control module on the 2020-2023 Mercedes-Benz GLS450 X167 — where it lives, replacement time, and what it takes to program a replacement. Modules marked dealer / factory tool won't work after a part swap alone — budget for programming.
Transmission Control Unit (TCU)2.5 hr R&Rsecurity gateway +1.0 hr▸ programming details
Aftermarket tool coverage varies by software version and vehicle build — treat "aftermarket tool" rows as "usually possible" and verify against your tool maker's coverage list before promising a customer. Spot a wrong location or hour? Tell us — corrections ship fast here.
Size-standard part numbers — verify your connector type before buying. Rear blades are model-specific; check the package's vehicle list.
Fuel economy figures are EPA data via fueleconomy.gov (median across matching trims). Performance figures are compiled estimates for the 2020 Mercedes-Benz GLS450 X167 3.0L I6 Turbo M256 and can vary by trim.
🔧 Database maintained under the daily editorial review of Chris Hackleman · Master Technician · 20+ years and Jeff Moore · Master Lexus & Toyota Mechanic · 20+ years.