Mercedes-Benz GLS: common faults & what to check

Which Mercedes-Benz GLS are you looking at?

2 generation variants documented. Pick the shape you recognise: the faults, the servicing and the timeline below all narrow to it. Free, no account needed.

Engine not documented

2 variants documented

2015–2019X1662015–2019X166Engine not documented for this generationEngine not documentedDocumented issues4 knownSee faults & what to checkKickTyres illustration, not a photograph.

2019–nowX1672019–nowX167Engine not documented for this generationEngine not documentedDocumented issues4 knownSee faults & what to checkKickTyres illustration, not a photograph.

Now narrow it to the engine

A 1.8 and a 2.4 of the same car do not fail the same way, so the faults below narrow to the engine as well as the year. Free, no account needed.

Start your free check on this Mercedes-Benz GLS

No plate needed. You get the risk score, the buyer's checklist and the guided inspection for the exact car you picked above — not the whole nameplate.

The checks worth doing on a Mercedes-Benz GLS before you hand over any money. Every documented fault is listed below, each with its own page.

• Serious issue

  Turn the steering from lock to lock while stationary with the engine running and check it is light and smooth with no warning message.

  See why

  X166 Electric Power Steering Connector Water Entry, Loss of Steering Assist Recall

  On late GL and early GLS X166 cars, the connector on the electric power steering control unit may not be sealed properly, so water can get in and cut the steering assist without warning, making the car very heavy to steer. Mercedes recalled it (NHTSA 17V-241 for 2016 GL and 2017 GLS; AU PRA 2017/16263 for X166 cars sold after 1 October 2015) to replace the connector and inspect the rack, replacing it if water damage is found. A related bulletin LI46.35-P-059241 covers power steering malfunction warnings with heavy steering. Check the recall has been done, because an unrepaired car can lose assist on the road.

  What you would notice

  • 'Power steering malfunction' warning
  • Steering suddenly heavy
  • Steering assist cutting in and out, often after rain or washing

  Typically $0–$4,500 to put right.

• Serious issue

  Ask the seller whether recall RC2978 (coolant pump and vacuum circuit) was done, and ring a Mercedes dealer with the VIN to confirm.

  See why

  Coolant pump leak into vacuum system, fire risk recall (GLS 400d 2.9 OM656 diesel, X167)

  On X167 GLS diesels with the 2.9 OM656 six cylinder built up to 2021, the vacuum-controlled coolant pump can leak coolant into the vacuum circuit. Coolant reaching the electrical switch valves can make them heat up over time and may start a fire. Mercedes recalled affected C, E, G, S, GLE and GLS diesels in Australia (RC2978, REC-005039) and the dealer fix is free. A car that has not had the recall done can also show unexplained coolant loss.

  What you would notice

  • Coolant level dropping with no visible leak
  • Coolant warning message
  • Burning or hot electrical smell from the engine bay

  Typically $0–$2,500 to put right.

• Costly issue

  On the test drive include stop-start traffic and a gentle slow-down to a stop, and feel for clunks as it changes down.

  See why

  9G-Tronic (725.0) Harsh Shifts, Shudder and Limp Mode (X166 GL350 / GLS)

  From around 2015 the GL350 BlueTEC, and every GLS350d and GLS500 after the 2016 facelift, use the 9G-Tronic 725.0 nine-speed automatic instead of the 7G-Tronic (the AMG 63 models kept a seven-speed). Owners report harsh or clunky low-speed shifts, hunting between gears, shudder when moving off, and occasional limp mode stuck in one gear. Mercedes bulletin LI27.00-P-060741 covers rough shifting, shudder and limp-home on the 725.0 and prescribes an oil level check, valve flushing, software updates and adaptation. Most cases are fixed with software and a correct oil level, but some need valve body or electrical work. There is no recall or extended warranty.

  What you would notice

  • Clunk or jolt on low-speed shifts, especially when slowing to a stop
  • Shudder when pulling away
  • Gear hunting on hills or in traffic
  • Transmission warning or limp mode stuck in one gear
  • Delay when selecting Drive or Reverse

  Typically $300–$6,500 to put right.

• Costly issue

  Turn the key to ignition on and check every warning light goes out once the engine is running.

  See why

  X166 OM642 BlueTEC AdBlue (SCR) Tank Heater, Heated Line and NOx Sensor Faults (GL350 / GLS350d)

  The X166 GL350 BlueTEC and GLS350d use the OM642 3.0 V6 diesel with an AdBlue (urea) SCR system to cut NOx. The heating element in the AdBlue tank drifts in resistance and fails (Mercedes bulletin LI49.20-P-056353 names model 166 with engine 642), the heated AdBlue delivery line can break internally, and the NOx sensors in the exhaust fail with age and heat. Any of these puts on the engine light and starts an AdBlue countdown that ends in a no-start after a set distance, and topping up AdBlue does not clear it. Parts must be coded and the SCR system re-initialised with a Mercedes-level scan tool after repair. There is no recall or extended warranty for this in NZ or AU.

  What you would notice

  • Engine warning light on
  • AdBlue or 'Check AdBlue' message on the dash
  • 'No start in X km' or 'Remaining starts' countdown
  • Message stays after AdBlue is topped up
  • Reduced power in some cases

  Typically $700–$4,500 to put right.

• Costly issue

  Look under the car where it has been parked for oil spots below the rear of the engine.

  See why

  X166 OM642 Oil Cooler Seal Leak in the Engine V (GL350 BlueTEC / GLS350d)

  The OM642 V6 diesel oil cooler sits in the valley between the cylinder banks under the intake manifold, and its rubber seals harden and leak. This continues from the earlier X164 GL onto the X166 GL350 BlueTEC and GLS350d, even with the later purple Viton seals; Mercedes issued bulletin LI18.30-P-055434 for oil traces at the oil cooler in the inner V of engine 642, listed against the 2015 GL350. Oil pools in the V and runs down the back of the engine onto the transmission bell housing, and once it starts the leak tends to speed up. The seals are cheap but the intake manifold and turbo pipework must come off, so most of the cost is labour. Ignored, it can drop the oil level far enough to damage the engine.

  What you would notice

  • 'Check oil level' or low oil message
  • Oil level dropping between services
  • Oil drips under the back of the engine
  • Burning oil smell after a drive
  • Oil wetness at the rear of the engine and the bell housing

  Typically $1,400–$3,000 to put right.

• Costly issue

  Ask whether the car has Active Curve System (a separate hydraulic reservoir with a dipstick cap in the engine bay, or check the option list).

  See why

  X166 Active Curve System Hydraulic Faults and Reservoir Oil Leak Recall

  Some X166 GL and GLS cars (option code 468, mostly with the On&Offroad package and on AMG models) have Active Curve System, a hydraulic anti-roll setup with its own pump, valve blocks and reservoir. Mercedes bulletin LI32.34-P-058736 covers 'ACTIVE CURVE SYSTEM malfunction' messages and low oil pressure codes caused by air in the system, with gushing noises when cornering and oil leaking from the reservoir. Separately, a recall (NHTSA 18V-272; AU PRA 2018/16986 for X166 cars sold 1 February 2012 to 31 August 2016) replaced the reservoir dipstick and cap because low oil could foam out of the vent onto hot engine parts and start a fire. A failed pump or valve block is expensive; a car without ACS is not affected.

  What you would notice

  • 'ACTIVE CURVE SYSTEM' malfunction message
  • Gushing or flowing noise from the front or rear when cornering
  • Body leaning more than expected in corners
  • Oily mess around the ACS reservoir in the engine bay

  Typically $300–$5,500 to put right.

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