The 2020 GLS (X167) represents Mercedes' third-generation full-size SUV with the new 3.0L inline-6 turbo or 4.0L twin-turbo V8. Early units are showing concerning powertrain issues—particularly catastrophic engine failures on the M256 inline-6 and transmission cooling problems—that are uncommon for vehicles this new.
Symptoms: Sudden metallic knocking or rattling from engine bay at idle or light throttle, Check engine light with misfire codes (P0300-series), Oil pressure warning light or low oil pressure message, Metal shavings in oil during routine changes, Catastrophic failure: loss of power, severe noise, engine seizure
Fix: This is a known weak point on early M256 engines—rod bearings fail prematurely, leading to piston and crankshaft damage. Repair requires complete engine disassembly or replacement. Short block replacement takes 25-35 hours; full engine replacement (long block) runs 18-24 hours. Many units under warranty, but out-of-pocket disasters exist. Some TSBs address oil pressure sensor recalibration, but actual bearing failure requires engine work.
Estimated cost: $15,000-30,000
9G-Tronic Transmission Oil Cooler Leaks
Common · medium severity
Typical onset: 40,000-80,000 mi
Symptoms: Transmission fluid dripping from undercarriage near front of transmission, Low transmission fluid warning on dash, Burnt smell or darker-than-normal ATF color, Intermittent harsh shifts or delayed engagement when fluid gets low
Fix: The cooler lines and cooler itself develop leaks at crimped connections or from external corrosion. Replacement involves dropping transmission pan, disconnecting lines, and replacing cooler assembly. Often done with full fluid service. Book time around 3-4 hours plus fluid refill and relearn procedure.
Estimated cost: $800-1,400
Transmission Mount Failure (Front and Rear)
Occasional · medium severity
Typical onset: 50,000-90,000 mi
Symptoms: Clunk or thud when shifting from Park to Drive or Reverse, Vibration felt through floor or steering wheel at idle in Drive, Excessive drivetrain movement visible when applying throttle from stop, Harsher shift feel as mount deteriorates
Fix: Hydraulic transmission mounts fail from heat and stress, especially in heavier GLS models. Front mount is easier (2-3 hours); rear mount requires more disassembly including exhaust and crossmember work (4-5 hours). Often replaced in pairs if one fails due to shared duty cycle. Requires lift and transmission support.
Estimated cost: $1,200-2,200
High-Pressure Fuel System Component Failures (Filter, Pump, Injectors)
Occasional · high severity
Typical onset: 60,000-100,000 mi
Symptoms: Hard starting or extended cranking, especially when cold, Rough idle or misfires at startup that clear after a few seconds, Loss of power under heavy acceleration, Fuel smell in cabin or engine bay, Check engine light with fuel rail pressure or injector codes
Fix: Direct-injection systems run 2,000+ psi and are sensitive to fuel quality. Filters clog prematurely if contaminated fuel is introduced; high-pressure pump can fail internally, and injectors develop leaks or carbon buildup. Fuel filter replacement is 1.5-2 hours; high-pressure pump is 4-6 hours (requires intake manifold removal on M256); injector replacement is 6-8 hours per bank. Misdiagnosis is common—always test fuel pressure first.
Estimated cost: $600-4,500
AIRMATIC Air Suspension Compressor and Strut Leaks
Common · medium severity
Typical onset: 60,000-100,000 mi
Symptoms: Vehicle sitting noticeably lower at one corner or entire side after sitting overnight, Suspension warning light and message 'AIRMATIC Malfunction: Visit Workshop', Compressor running excessively (audible whirring near rear wheel well), Rough ride quality or inability to raise/lower suspension on command
Fix: AIRMATIC struts develop leaks at the upper air sleeve or lower seal; compressor wears out from overwork compensating for leaks. Strut replacement is 3-4 hours per corner; compressor replacement is 2-3 hours and located behind rear bumper driver side. Delay leads to compressor burnout, doubling the cost. Always inspect all four struts when one fails—they tend to leak in groups.
Estimated cost: $1,800-3,500
Coolant Thermostat and Auxiliary Water Pump Failures
Occasional · medium severity
Typical onset: 50,000-90,000 mi
Symptoms: Check engine light with P0597 (thermostat heater control) or coolant temp sensor codes, Engine takes unusually long to reach operating temperature, Heater blows cold air intermittently at idle, Overheating in stop-and-go traffic or at idle (auxiliary pump failure)
Fix: Electronically-controlled thermostats fail stuck-open or with heater element faults; auxiliary coolant pumps (used for turbo/transmission cooling and cabin heat) seize or leak. Thermostat replacement is 2-3 hours; aux pump is 1.5-2.5 hours depending on location (multiple pumps in system). Use OE parts only—aftermarket pumps fail quickly.
Estimated cost: $600-1,200
Owner tips
Change transmission fluid every 40,000 miles regardless of 'lifetime fill' claims—heat and stress degrade ATF quickly in heavy SUVs
Use only Top Tier fuels and add fuel system cleaner every 10,000 miles to prevent direct-injection carbon buildup and high-pressure fuel system issues
Monitor oil consumption closely on M256 engines—consuming more than 1 quart per 1,500 miles can indicate early bearing wear
Inspect AIRMATIC struts annually for moisture or oil residue around the air sleeve—early detection prevents compressor damage
If buying used, pull engine oil sample and send for analysis to check bearing material before purchase—cheap insurance against $20k engine failures
Avoid early M256 inline-6 models unless full engine replacement history is documented—the catastrophic failure risk is too high for a 4-5 year old luxury SUV; V8 variants are safer but still demand religious maintenance.
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 battery required; located under hood on right side; battery registration required after replacement
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Every control module on the 2018-2020 Mercedes-Benz GLS — 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.
Airmatic / Active Body Control Module (ABC/ADS)2.5 hr R&Rsecurity gateway +1.5 hr▸ programming details
📍 Front left engine bay or under vehicle near front axle
🔧 Xentry Diagnostics + SCN online
⚠️ VIN-locked. Requires ride height calibration, damper adaptation, and level sensor learning. System bleed may be required.
Transmission Control Unit (TCU)2.5 hr R&Rsecurity gateway +1.0 hr▸ programming details
📍 Transmission housing or adjacent to transmission
🔧 Xentry Diagnostics + SCN online
⚠️ VIN-locked, requires SCN coding. Transmission fluid change often required during R&R.
Signal Acquisition and Actuation Module - Roof (SAM-Roof)2.0 hr R&Rsecurity gateway +1.0 hr▸ programming details
⚠️ Separate modules for driver and passenger. Requires position calibration after replacement.
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 GLS 4.0L Turbo V8 and can vary by trim.
🔧 Database maintained under the daily editorial review of Chris Hackleman · Master Technician · 20+ years and Jeff Moore · Master Mechanic · 20+ years.