OM656 vs D4/D5: which engine is more reliable?

According to our risk index, the OM656 carries less risk than the D4/D5: 39/100 (moderate risk) versus 64/100 (high risk). The OM656 has 2 documented serious faults, the main one being «Leak in the vacuum-driven coolant pump (fire risk)»; the D4/D5 has 2 documented serious faults, the main one being «Auxiliary belt breakage that takes the timing belt with it (5-cylinder)».

Risk index

OM656

Moderate risk · 2 serious faults

Lower risk

Risk index

D4/D5

High risk · 2 serious faults

The index (0–100) weights the severity and frequency of the three most serious documented faults. How it is calculated

Reviewed on 9 October 202616 sources How we work

Is the OM656 reliable?

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]}

See the 4 of the OM656 faults

Is the D4/D5 reliable?

The Volvo D5/D4 covers two different engines: the classic 5-cylinder (2001-2016, D5244T/D5204) and the 4-cylinder VEA (D4204T, from 2013). On the former, several owners document the auxiliary belt breaking and taking the timing belt with it, destroying the engine; on the VEA the most common problems are the EGR, the DPF and oil consumption that can damage the turbo from 90,000-150,000 km. Neither is an engine to buy without checking its service history.^{[9][10][11][12]}

See the 6 of the D4/D5 faults

Side-by-side comparison

Comparison of OM656 and D4/D5
Item	OM656	D4/D5
Technical data
Type	Engine	Engine
Fuel	Diesel	Diesel
Displacement	2,925 cm³	—
Timing	Chain	Timing belt (dry)
Power	272–367 hp	120–235 hp
Production	since 2017^[1][2][13]	2001–2024^[9][10][14]
Reliability
Risk index (what is it?)	39Moderate risk	64High risk
Documented faults	4	6
Of high or critical severity	2	2
Worst severity	Critical severity	Critical severity
Main faults	Critical severity: Leak in the vacuum-driven coolant pump (fire risk) Medium severity: NOx sensor failure and AdBlue (SCR) system warnings Medium severity: EGR valve clogged with soot after short journeys	Critical severity: Auxiliary belt breakage that takes the timing belt with it (5-cylinder) High severity: High oil consumption from 90,000-150,000 km that harms the turbo (VEA) Medium severity: Clogged EGR valve or EGR cooler leaking into the intake
Most expensive documented repair	No reliable data	No reliable data
Which versions to buy
BuyRecommended versions	OM656 3.0 diesel with electric assistance (D 30, from 2023)^[4][5][7]	None
With cautionVersions to be cautious about	OM656 2.9 diesel 350 d (OM 656 D 29 R SCR, from 2017)^[3][4][5] OM656 2.9 diesel 400 d (OM 656 D 29 SCR, from 2017)^[3][4][5][7]	2.4/2.0 D3/D4/D5 5-cylinder (D5244T/D5204, 2001-2016)^[11][15] 2.0 D2/D3/D4/D5 VEA 4-cylinder (D4204T, 2013-2024)^[11][12][16]
AvoidVersions to avoid	None	None

Severity of each fault: green, low; amber, medium; red, high; dark red, critical. How we rate them

Cars that each one is fitted to

OM656

See the 11 models with the OM656

D4/D5

See the 14 models with the D4/D5

Other comparisons

With the OM656

With the D4/D5

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Sources

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

Mercedes-Benz OM656 engineWikipedia · Other source · consulted 9 October 2026
Mercedes-Benz OM 656Wikipedia (alemán) · Other source · consulted 9 October 2026 · in German
Rückruf für OM 656 und OM 654 Motoren wegen KühlmittelpumpeMBpassion · Motoring press · published 9 December 2021 · consulted 9 October 2026 · in German
Mercedes-Rückruf: Kühlmittelprobleme bei DieselautosAUTOHAUS · Motoring press · published 5 January 2022 · consulted 9 October 2026 · in German
DVSA – base de llamadas a revisiónDVSA (Reino Unido) · Other source · consulted 9 October 2026
Mercedes-Benz GLC (X253): Alle typischen Probleme & MängelGebrauchtwagenberater · Motoring press · consulted 9 October 2026 · in German
Mercedes OM656 Diesel: Häufige Probleme, Ursachen & Tipps zur Vermeidung von MotorschädenMBpassion · Motoring press · published 19 February 2026 · consulted 9 October 2026 · in German
Mercedes-Benz OM656 Engine Guide: Reliability, Common Problems & Tuning PotentialEuro Premium Parts · Technical documentation · consulted 9 October 2026
Volvo Engine ArchitectureWikipedia · Other source · consulted 27 September 2026
Volvo D5: 185 CV para el S60, V70, XC70 y XC90Autopista.es · Motoring press · published July 2005 · consulted 27 September 2026 · in Spanish
Fiabilité et pannes Volvo XC60 (2008-2017)Fiches-auto.fr · Other source · consulted 27 September 2026 · in French
Fiabilité et pannes Volvo V60Fiches-auto.fr · Other source · consulted 27 September 2026 · in French
Mercedes-Benz S-Class (W222)Wikipedia · Other source · consulted 9 October 2026
Volvo XC90Wikipedia · Other source · consulted 27 September 2026 · in Spanish
Fiabilité et pannes Volvo S60Fiches-auto.fr · Other source · consulted 27 September 2026 · in French
Fiabilité et pannes Volvo XC60 (2017-)Fiches-auto.fr · Other source · consulted 27 September 2026 · in French