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OM656 engine: faults and reliability

Mercedes-Benz OM656 (3.0 diésel, 6 cilindros en línea) · Mercedes-Benz

The OM656 is the roughly 3.0-litre in-line six-cylinder diesel with which Mercedes-Benz replaced the OM642 V6 from 2017 in the S-Class, E-Class, CLS, GLE, GLS and G-Class (350 d, 400 d and 450 d). It has few documented faults: the known campaign is the coolant pump one of 2021-2022, and the timing chain recall that circulates belongs to the OM654. Guides warn about the AdBlue/SCR system, the EGR and, in isolated cases, the roller rocker arms, without figures.[1][2][3][4][5][6][7][8]

Risk index
39
Moderate risk · what it is
Documented faults
4
2 of high or critical severity
Timing
Chain
Technical data[1][2][9]
Production
since 2017
11 models on this site

13 sourcesHow we work

Which OM656 versions should you buy?

Our verdict for each version, based on the documented faults.

  • OM656 2.9 diesel 350 d (OM 656 D 29 R SCR, from 2017)

    since 2017 · 272–286 hp · OM 656 D 29 R SCR, D29T R SCR

    With caution

    Worth buying if you confirm with the chassis number that the coolant pump campaign (cars built between 2017 and October 2021) has been carried out. We have not found serious, widespread mechanical faults, but the engine's history is short and the sources are scarce.[3][4][5]

  • OM656 2.9 diesel 400 d (OM 656 D 29 SCR, from 2017)

    since 2017 · 330–340 hp · OM 656 D 29 SCR, D29T SCR

    With caution

    Same case as the 350 d version: check that the coolant pump campaign (built from 2017 to October 2021) has been carried out. The sources do not indicate that the 400 d fails more or less than the 350 d.[3][4][5][7]

  • OM656 3.0 diesel with electric assistance (D 30, from 2023)

    since 2023 · 313–367 hp · D30T R M, D30T M

    Buy

    We have not found faults or campaigns specific to this version, and it falls outside the build period of the coolant pump campaign (until October 2021). It is recent and its history is short, so the absence of data is not the same as a guarantee.[4][5][7]

Buy: No known serious faults, or they have already been corrected, in this version. With caution: Worth buying if something specific is verified: history, a replaced part or a completed campaign. Avoid: High risk of a serious fault that is hard to check for or prevent.

Known faults of the OM656

Sorted from most to least important (severity × frequency).

  1. 1Critical severityFrequency: OccasionalCoolingLeak in the vacuum-driven coolant pump (fire risk)Mercedes-Benz recalled OM654 and OM656 diesels built between 2017 and October 2021 because the vacuum-driven coolant pump can lose its seal and, in individual cases, a fire risk cannot be ruled out. The remedy: replace an electric valve and update the software.
  2. 2Medium severityFrequency: OccasionalEmissions controlNOx sensor failure and AdBlue (SCR) system warningsTwo OM656-specific guides place the SCR/AdBlue system among the engine's sensitive points (one of them cites the NOx sensors and the AdBlue injector), especially with short journeys and poor-quality fuel. There are no public failure figures: we rate it occasional, not a series defect.
  3. 3Medium severityFrequency: OccasionalEmissions controlEGR valve clogged with soot after short journeysTwo OM656 guides include a dirty or clogged EGR valve among the typical problems, linked to short, urban journeys: loss of power and warning messages. A timely cleaning is usually enough; if the cooler is damaged, the repair gets more expensive. No failure-rate figures.
  4. 4High severityFrequency: RareTimingFaulty roller rocker arms and Camtronic solenoid valve failuresMBpassion reports isolated cases, in cars up to 2022, of faulty roller rocker arms that can leave metal shavings in the oil circuit, as well as failures of the Camtronic system's solenoid valves. It is a single source with no figures: a warning, not a proven fault.

Severity colours: low, medium, high, critical. How we rate them

By part

Compare each fault with the same part on other engines: symptoms, cost and affected models.

Symptoms of these faults

What the OM656 is

The OM656 is an in-line six-cylinder diesel that Mercedes-Benz presented in 2017 as the successor to the OM642 V6 and which is part of a modular family alongside petrol engines. According to Wikipedia, it has an aluminium block and head, a cylinder-wall coating applied by wire spraying (Nanoslide), four valves per cylinder, two chain-driven camshafts with switchable exhaust cams, and a cylinder spacing of 90 mm. It is built by MDC Power in Kölleda (Germany).[1][2]

Charging is two-stage (BorgWarner turbos), injection is common rail of up to 2,700 bar with a stepped combustion chamber, and aftertreatment combines cooled high- and low-pressure EGR, an oxidation catalyst, AdBlue injection and a particulate filter with SCR coating plus an SCR catalyst, according to German Wikipedia. There are two power levels in the 2.9: 272-286 hp (350 d) and 330-340 hp (400 d).[1][2]

It debuted in the facelifted W222 S-Class, whose production started in July 2017, and then came to the CLS (March 2018), the E-Class (400 d from April 2018 and 350 d from September), the GLE (December 2018), the G-Class (350 d from January 2019), the GLS (October 2019), the GLC 400 d (July 2019) and the W223 S-Class (September 2020), according to German Wikipedia. Since 2023 there has been a 2,989 cm³ version with 15 kW of electric assistance in the 450 d variants (S, GLE, GLS, GLC X254, E-Class W214 and G-Class W465) and in the S 350 d and GLS 350 d, according to English Wikipedia.[1][2][9][10][11]

On reliability, the only documented official campaign is the one for the vacuum-driven coolant pump (OM654 and OM656 diesels built between 2017 and October 2021). The timing chain recall (January 2016 to February 2021) cited by some websites affects only the four-cylinder OM654. MBpassion considers it very reliable if looked after and maintained with quality parts, and the French testimony site fiches-auto records hardly any reviews of this engine (one S 350 d with 286 hp, the output of this engine, with no declared faults, and none for a 400 d in the E-Class or S-Class).[3][4][5][6][7][12][13]

Manufacturer
Mercedes-Benz (Daimler)
Family
OM656
Fuel
Diesel
Displacement
2,925 cm³
Cylinders
6
Fuel system
Twin-turbo
Injection
Common rail
Timing
Chain
Power
272–367 hp
Peak torque
750 Nm
Production
since 2017
Makes
Mercedes-Benz

Technical data[1][2][9]

What changed between versions

  1. since 2017

    OM656 2.9 diesel 350 d (OM 656 D 29 R SCR, from 2017)

    Lower-output version of the 2,925 cm³ 2.9: 272 hp in the GLE 350 d and 286 hp (210 kW) in the S-Class, E-Class, CLS, G-Class and GLS, with 600 N·m of torque. It debuted in the facelifted W222 S-Class (production from July 2017) and then went into the CLS, E-Class, GLE, G-Class, GLS and the W223 S-Class.[1][2][9]

  2. since 2017

    OM656 2.9 diesel 400 d (OM 656 D 29 SCR, from 2017)

    Higher-output version of the same 2.9: 330 hp (243 kW) in the GLC, GLE, GLS and G-Class, and 340 hp (250 kW) in the S-Class, E-Class and CLS, with 700 N·m of torque. It shares the block and basic mechanics with the 350 d.[1][2]

  3. since 2023

    OM656 3.0 diesel with electric assistance (D 30, from 2023)

    Displacement increased to 2,989 cm³. According to English Wikipedia, the 350 d version gives 230 kW (about 313 hp) and the 450 d 270 kW (about 367 hp), plus 15 kW of electric assistance that the source does not explain; 750 N·m of torque in the 450 d. It powers the S-Class, GLS, GLE, GLC (X254), E-Class (W214) and G-Class (W465).[1][10][11]

Cars fitted with the OM656

11 models sold in Europe. Open each one to see all their faults and the buying checklist.

Mercedes-Benz

Compare the OM656

Maintenance parts for the OM656

The parts replaced at services and when the timing belt or chain is changed, searched for this engine. Confirm the part number with the registration plate or VIN.

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Sources

Every fact on this page links to its source. If you find an error, let us know.

  1. Mercedes-Benz OM656 engineWikipedia · Other source · consulted 9 October 2026
  2. Mercedes-Benz OM 656Wikipedia (alemán) · Other source · consulted 9 October 2026 · in German
  3. Rückruf für OM 656 und OM 654 Motoren wegen KühlmittelpumpeMBpassion · Motoring press · published 9 December 2021 · consulted 9 October 2026 · in German
  4. Mercedes-Rückruf: Kühlmittelprobleme bei DieselautosAUTOHAUS · Motoring press · published 5 January 2022 · consulted 9 October 2026 · in German
  5. DVSA – base de llamadas a revisiónDVSA (Reino Unido) · Other source · consulted 9 October 2026
  6. Mercedes-Benz GLC (X253): Alle typischen Probleme & MängelGebrauchtwagenberater · Motoring press · consulted 9 October 2026 · in German
  7. Mercedes OM656 Diesel: Häufige Probleme, Ursachen & Tipps zur Vermeidung von MotorschädenMBpassion · Motoring press · published 19 February 2026 · consulted 9 October 2026 · in German
  8. Mercedes-Benz OM656 Engine Guide: Reliability, Common Problems & Tuning PotentialEuro Premium Parts · Technical documentation · consulted 9 October 2026
  9. Mercedes-Benz S-Class (W222)Wikipedia · Other source · consulted 9 October 2026
  10. Mercedes-Benz G-ClassWikipedia · Other source · consulted 9 October 2026
  11. Mercedes-Benz E-Class (W214)Wikipedia · Other source · consulted 9 October 2026
  12. Fiabilité Mercedes Classe E (2016-2023)Fiches-auto.fr · Motoring press · consulted 9 October 2026 · in French
  13. Fiabilité Mercedes Classe S (2013-2021)Fiches-auto.fr · Motoring press · consulted 9 October 2026 · in French