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Mercedes-Benz2013–2023 Critical issues documented

Mercedes-Benz GLS X166: common faults & what to check

The X166 is the Mercedes-Benz GLS built 2015–2019. Of the 10 faults documented for the GLS, 4 apply to this generation, each with its symptoms, what to inspect and a typical repair cost. Free, no account needed.

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10

Documented faults

Critical

Most serious

$300–$4,500

Typical repair

6

Systems covered

You're looking at the X166

Everything below is narrowed to this variant. Pick a different one to change it.

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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.

Which engine is in it? These cover every year of this model.

GLS350dGLS400GLS450GLS500GLS63

A 2015 Mercedes-Benz GLS is the X166 generation (2015 to 2019).

We cannot narrow the engine list to this year, so every engine this model was sold with is offered.

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Mercedes-Benz GLS X166Engine not picked yet

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Can't confirm for this car

Mercedes-Benz GLS · 2015

Narrowed to the year. No engine is recorded for this vehicle. Pick an engine to see which faults apply.

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matched on year only

See all 4 issues for this oneDocumented for the exact year and engine you picked.
Body, brakes, suspension & interior1

01X166 Airmatic Front Air Strut Leak, Valve Block and Compressor Failure

2013–2019SuspensionRead the full fault: symptoms, where to look, what to ask the seller
Depends on the exact engine — 3 moreDocumented for a specific engine we could not confirm from what you picked. Check the engine code on the car.
Engine, transmission & cooling1

01X166 OM642 BlueTEC AdBlue (SCR) Tank Heater, Heated Line and NOx Sensor Faults (GL350 / GLS350d)

Turn the key to ignition on and check every warning light goes out once the engine is running.diesel2013–2019EngineRead the full fault: symptoms, where to look, what to ask the seller
Body, brakes, suspension & interior1

01X166 Active Curve System Hydraulic Faults and Reservoir Oil Leak Recall

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).2013–2019Suspension
OccasionalSuspension $300–$5,500 to fix

Can't confirm for this car

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.

Symptoms to 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

What to check

  1. 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).
  2. If fitted, look for oil wetness around the reservoir and on the ground underneath.
  3. Watch for an Active Curve message on the dash, and on the test drive corner firmly at low speed and listen for gushing noises and excess body lean.
  4. Ask for proof that the Active Curve reservoir recall was completed.
  5. If a workshop inspection is possible: read the ACS control unit for codes such as P1C9C00 or P1C9E00, check the oil level and fluid type (Pentosin CHF 11S), and run the bleed routine to see if the fault returns.

Where to look: Engine bay (ACS oil reservoir and pump), valve blocks and roll-control actuators at the front and rear axles

Ask the seller

  • “Does this car have Active Curve System?”
  • “Has the Active Curve reservoir recall been done?”
  • “Has an Active Curve warning ever come up?”
Fluid leaks1

01X166 OM642 Oil Cooler Seal Leak in the Engine V (GL350 BlueTEC / GLS350d)

Look under the car where it has been parked for oil spots below the rear of the engine.diesel2013–2019Fluid LeaksRead the full fault: symptoms, where to look, what to ask the seller
Other Mercedes-Benz GLS engines and years6 documented issues that do not apply to the one you picked.
Engine, transmission & cooling2

01Coolant pump leak into vacuum system, fire risk recall (GLS 400d 2.9 OM656 diesel, X167)

Ask the seller whether recall RC2978 (coolant pump and vacuum circuit) was done, and ring a Mercedes dealer with the VIN to confirm.diesel2019–2021Cooling
OccasionalCooling $0–$2,500 to fix

Does not apply

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.

Symptoms to notice

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

What to check

Ask the seller whether recall RC2978 (coolant pump and vacuum circuit) was done, and ring a Mercedes dealer with the VIN to confirm. At the viewing, check the coolant level in the expansion tank against the marks with the engine cold, and sniff the engine bay for a hot electrical or sweet coolant smell after the test drive. Ask whether coolant has been topped up often. If a workshop inspection is possible: have the vacuum lines and switch valves checked for coolant contamination.

Where to look: Engine bay, coolant pump, vacuum lines and switch valves

Parts involved: Water pump.

Ask the seller

  • “Has recall RC2978 for the coolant pump been completed?”
  • “Has the car ever needed coolant topped up between services?”
  • “Any coolant or overheating warnings in the past?”

029G-Tronic (725.0) Harsh Shifts, Shudder and Limp Mode (X166 GL350 / GLS)

On the test drive include stop-start traffic and a gentle slow-down to a stop, and feel for clunks as it changes down.2017–2019TransmissionRead the full fault: symptoms, where to look, what to ask the seller
Electrical2

01X166 Electric Power Steering Connector Water Entry, Loss of Steering Assist Recall

Turn the steering from lock to lock while stationary with the engine running and check it is light and smooth with no warning message.2017Electrical
OccasionalElectrical $0–$4,500 to fix

Does not apply

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.

Symptoms to notice

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

What to check

  1. Turn the steering from lock to lock while stationary with the engine running and check it is light and smooth with no warning message.
  2. On the test drive watch for any power steering warning.
  3. Ask for the service book or a dealer printout showing the power steering connector recall was completed.
  4. Ask whether the steering has ever gone heavy.
  5. If a workshop inspection is possible: have a Mercedes dealer run the VIN to confirm recall completion, read the power steering control unit for fault codes, and inspect the connector and rack for water entry or corrosion.

Where to look: Electric power steering rack and its control unit on the front subframe, low behind the front wheels

Parts involved: Steering rack.

Ask the seller

  • “Has the power steering recall been done, and can you show proof?”
  • “Has the steering ever gone heavy or shown a warning?”

0248-volt mild-hybrid wiring harness recall — fire risk

2019–2023Electrical
OccasionalElectrical $0–$400 to fix

Does not apply

The 48-volt mild-hybrid system fitted to these cars was the subject of recall action, including in Australia, after a manufacturing defect meant the threaded nut securing the 48V wiring harness under the driver's seat may not have been tightened properly. A loose high-current connection is a fire risk, which is why it was recalled rather than handled as a service item. The work is free, but a car that has been privately imported or has passed through several owners may never have had it carried out, and there is nothing visible from the driver's seat to tell you either way. The VIN is the only reliable answer.

Symptoms to notice

  • No recall paperwork anywhere in the car's history
  • Burning or hot-electrical smell from under the driver's seat
  • Intermittent electrical faults or a warning about the mild-hybrid system
  • Car privately imported, with no local dealer history

What to check

  1. Take the VIN from the windscreen or the driver's door jamb and ask a Mercedes-Benz dealer whether any recall remains open against it — free, and the only way to know.
  2. Ask the seller for dealer service records and any recall letters, and pay attention to whether the car has full local history or was imported privately.
  3. Sit in the driver's seat with the car running and note any hot-electrical or burning smell.
  4. Ask whether any mild-hybrid or electrical warning has ever appeared.
  5. If a workshop inspection is possible: ask for the stored fault memory to be read across the electrical modules.

Where to look: 48V wiring harness connection beneath the driver's seat

Ask the seller

  • “Has the 48V harness recall been carried out on this VIN?”
  • “Do you have dealer service records or recall letters?”
  • “Was the car sold new here, or imported?”
Body, brakes, suspension & interior2

01AIRMATIC air strut leaks — a corner that sits down overnight

The X167 GLS rides on air, and leaking air struts are the reliability concern owners talk about most.2019–2023SuspensionRead the full fault: symptoms, where to look, what to ask the seller

02AIRMATIC warning that is SOFTWARE, not a broken strut (C220C04 / C220C06)

Check the two things together: is there an AIRMATIC message, and does the car actually behave?2019–2023Suspension
OccasionalSuspension $200–$600 to fix

Does not apply

This one is here to save a buyer several thousand dollars. Mercedes issued a technical service bulletin for the W167 and X167 platform covering an AIRMATIC fault message on the dash caused by an internal software error in the suspension control unit, logging codes C220C04 or C220C06. The remedy is a software update — replacing struts will NOT clear it. Plenty of owners have had suspension parts fitted chasing a warning that was never mechanical, and a seller may honestly tell you the car has "had suspension trouble" when what it needed was a laptop. So an AIRMATIC message on a car that actually sits level, rides normally and holds its height overnight is a very different situation from a corner on the ground.

Symptoms to notice

  • AIRMATIC or suspension warning message with the car sitting perfectly level
  • Warning that clears on a restart and later returns
  • Ride quality and height that are entirely normal despite the message
  • History of suspension parts fitted without the warning going away

What to check

  1. Check the two things together: is there an AIRMATIC message, and does the car actually behave?
  2. Look at the ride height at all four corners after it has stood, listen for a compressor that will not stop, and drive it over a rough surface.
  3. A car that sits level, holds height and rides properly while showing the message is pointing at the software fault rather than a strut.
  4. Ask the seller what has already been replaced and whether the warning went away afterwards — parts fitted with no improvement is the giveaway.
  5. If a workshop inspection is possible: ask specifically for the stored codes, and whether C220C04 or C220C06 are present, before agreeing to any suspension work.

Where to look: AIRMATIC suspension control unit — the fault is in its software, not at the wheels

Parts involved: Air suspension strut, Shock absorber / strut.

Ask the seller

  • “Is there an AIRMATIC warning, and has the software update been applied?”
  • “What suspension parts have been replaced, and did the warning stop?”
  • “Does the car sit level after standing overnight?”

Mercedes-Benz GLS faults in detail

Narrowed to the Mercedes-Benz GLS · 2015. Each of these has its own page: the symptoms, where to look, what to ask the seller and what the repair costs. Free, no account needed.

X166 Airmatic Front Air Strut Leak, Valve Block and Compressor FailureMajor · SuspensionX166 OM642 BlueTEC AdBlue (SCR) Tank Heater, Heated Line and NOx Sensor Faults (GL350 / GLS350d)Major · EngineX166 OM642 Oil Cooler Seal Leak in the Engine V (GL350 BlueTEC / GLS350d)Major · Fluid Leaks

Other Mercedes-Benz models

Mercedes-Benz C-Class41 documented faultsMercedes-Benz A-Class23 documented faultsMercedes-Benz E-Class23 documented faultsMercedes-Benz ML10 documented faultsMercedes-Benz B-Class8 documented faultsMercedes-Benz GLB8 documented faults

Mercedes-Benz GLS buyer questions

Is the Mercedes-Benz GLS reliable?

Like any used car, it depends on the specific example and its service history. Across the 10 documented faults for the Mercedes-Benz GLS, the recurring problem areas are suspension, electrical, and cooling. A Mercedes-Benz GLS with a full service history and no signs of the issues listed on this page can be a sound buy but each one needs to be checked individually before you commit.

What are the most common problems with a used Mercedes-Benz GLS?

We've catalogued 10 known faults for the Mercedes-Benz GLS, covering suspension, electrical, and cooling. All 10 are written out in full on this page, with the symptoms to notice, what to inspect, and typical repair costs for each. Free to read, no account needed.

How much do repairs cost on a Mercedes-Benz GLS?

Most single repairs on the Mercedes-Benz GLS fall between $300–$4,500 at NZ independent-workshop rates. Some of the 10 documented faults cost less and a few cost considerably more, and no one car needs them all. Knowing which ones to look for before you view is what lets you negotiate the price down or walk away.

Should I get a pre-purchase inspection on a Mercedes-Benz GLS?

Yes. A Mercedes-Benz GLS inspection focused on its known weak points is the single best way to avoid an expensive mistake. KickTyres gives you a guided, model-specific inspection plus a market valuation and negotiation figure for $24.90 far less than the cost of missing one of the faults on this page.