2020 MERCEDES-BENZ GLS

Typical onset: 30,000-70,000 mi

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

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

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

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

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

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