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Mercedes S 350d/400d SPECIALISTS

Mercedes Benz S Class 350d/400d Engine Replacement Compare Prices & Get Quotes

Get quotes today from trusted UK Mercedes Benz S Class 350d and 400d engine specialists. Compare prices on reconditioned, rebuilt and used engines for the 2.9-litre OM656 inline-six diesel - supply & fit available nationwide.

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Mercedes Benz S Class 350d/400d Engine
from£2,400 to £4,900
  • Usedfrom £1,500
  • Reconditionedfrom £2,400
  • Common codesOM656.929, OM656.932, OM656.964
Mercedes Benz s-class 350d/400d Used Engine — OM656.929Mercedes Benz s-classUsed Engine — OM656.929Mercedes Benz s-class 350d/400d Reconditioned Engine — OM656.929Mercedes Benz s-classReconditioned Engine — OM656.929Mercedes Benz s-class 350d/400d Rebuilt Engine — OM656.929Mercedes Benz s-classRebuilt Engine — OM656.929

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3 Simple Steps to Finding the Right Replacement Engine

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Variant History

Mercedes Benz S Class 350d/400d Engine History & Specs at a Glance

The S 350d and S 400d launched with the facelift W222 generation in 2017, powered by the all-new OM656 3.0-litre inline-six diesel - the most powerful series-production car diesel engine ever offered by Mercedes-Benz. The OM656 replaced the OM642 V6 diesel with a straight-six architecture featuring stepped-bore combustion, twin sequential turbochargers, and advanced emissions technology. The S 400d, with 340hp and 700Nm of torque, set a new benchmark for diesel performance in a luxury saloon. Both variants continued into the W223 generation, representing the pinnacle of Mercedes diesel engineering.

Mercedes Benz S Class 350d/400d
  1. 2017

    OM656.929, 3.0L inline-six diesel, 286hp @ 3,400-4,600rpm (S 350d, W222 facelift)

  2. 2017

    OM656.929, 3.0L inline-six diesel, 340hp @ 3,600-4,200rpm (S 400d, W222 facelift)

  3. 2018

    OM656.929, 3.0L inline-six diesel, 340hp (S 400d 4MATIC)

  4. 2020

    W223 generation launch; OM656 continues with refinements

  5. 2021

    2023 - OM656.929, 2.9L inline-six diesel, 282hp (S 350d, W223)

  6. 2024

    present - OM656 continues as the flagship diesel for S-Class

Every engine fitted to the Mercedes Benz S Class 350d and 400d is covered below, with UK rebuilt pricing and known failure points - from OM656 EGR cooler cracking to vacuum pump and camshaft rocker wear issues.

Engine Codes & Pricing

Mercedes Benz S Class 350d/400d Engine Codes Specifications & Replacement Cost

Find your exact engine code, specs and UK replacement cost guide.

OM656.929 — 2.9L Inline-Six Diesel (2017-present) engine

OM656.929 — 2.9L Inline-Six Diesel (2017-present)

Power Output
286 hp (210 kW) @ 3,400-4,600 rpm (S 350d); 340 hp (250 kW) @ 3,600-4,200 rpm (S 400d)
Torque
600 Nm (S 350d); 700 Nm (S 400d)
Years Fitted
2017-present
Displacement
2,925 cc
Cylinders
Inline 6, DOHC, 24-valve, twin-turbocharged
Engine Code
OM 656.929

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Common Problems

Mercedes Benz S Class 350d/400d Engine Problems Repair Cost vs Replacement

The OM656 inline-six diesel is Mercedes' most advanced diesel engine, delivering exceptional performance and refinement. However, as with any complex modern diesel, it has well-documented issues that can lead to significant repair costs. EGR system clogging, vacuum pump failures, and camshaft wear are the most commonly reported problems. Every rebuilt engine supplied through EnginesMarket includes upgraded components addressing these original failure points, plus a minimum 12-month unlimited-mileage warranty.

Common failure points and repair options

OM656 EGR System Clogging & Cooler Issues

Affected
Mercedes S 350d, S 400d (OM656.929, 2017-present)
Typical failure mileage
40,000-80,000 miles
Root cause
The OM656 features both cooled low-pressure and high-pressure EGR systems. Carbon deposits can accumulate over time, leading to reduced performance, increased emissions, and warning lights on the dashboard. The EGR valve and cooler are prone to clogging with soot and combustion by-products.

Repair options

Repair tierDealer priceSpecialist price
Minor - EGR system cleaning£400-£700£250-£450
Intermediate - EGR cooler replacement£1,200-£2,000£750-£1,200
Full Replacement - Rebuilt OM656 engine£7,500-£11,000£2,800-£4,900
Our recommendation
EGR clogging on the OM656 is primarily an emissions-system maintenance issue. If caught early, cleaning the EGR system at a specialist is a cost-effective solution. If the EGR cooler requires replacement, specialist pricing of £750-£1,200 is significantly lower than dealer rates. Regular use of quality diesel and occasional motorway driving can help prevent carbon buildup.
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OM656 Vacuum Pump Failure

Affected
Mercedes S 350d, S 400d (OM656.929, 2017-2021)
Typical failure mileage
25,000-40,000 miles
Root cause
A leak may develop between the coolant pump and the vacuum circuit, allowing coolant to enter the vacuum system. This can affect components and lead to an increase in temperature, potentially causing a fire. Mercedes recalled over 800,000 vehicles worldwide with OM654 and OM656 diesel engines over this issue.

Repair options

Repair tierDealer priceSpecialist price
Recall Remedy - Coolant pump vacuum leak repair£0 (if recall applies)N/A
Intermediate - Vacuum system inspection & repair£500-£1,000£350-£700
Full Replacement - Rebuilt OM656 engine£7,500-£11,000£2,800-£4,900
Our recommendation
If your S 350d or S 400d is subject to the coolant pump vacuum leak recall, have the work performed at a Mercedes dealer free of charge. If symptoms persist or the recall doesn't apply, specialist inspection is essential. On an S-Class worth £25,000-£60,000, preventative attention to this issue is always the better investment.
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OM656 Camshaft & Rocker Wear

Affected
Mercedes S 350d, S 400d (OM656.929, 2017-present)
Typical failure mileage
80,000-100,000 miles
Root cause
OM654 and OM656 engines can suffer from camshaft and rocker wear, typically around 100,000 miles, sometimes less. Cam rocker wear can manifest as a popping sound from the intake. This is a relatively expensive repair job requiring specialist attention.

Repair options

Repair tierDealer priceSpecialist price
Diagnostics - Camshaft inspection£300-£500£150-£300
Intermediate - Camshaft & rocker replacement£2,000-£3,500£1,200-£2,000
Full Replacement - Rebuilt OM656 engine£7,500-£11,000£2,800-£4,900
Our recommendation
Camshaft and rocker wear on the OM656 is a known high-mileage concern. If you hear unusual noises from the intake or experience rough running, have the engine inspected immediately. Replacing camshafts and rockers at a specialist is significantly cheaper than dealer rates. On an S-Class worth £25,000-£60,000, preventative attention is always the better investment.
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OM656 AdBlue System Failure

Affected
Mercedes S 350d, S 400d (OM656.929, 2017-present)
Typical failure mileage
30,000-60,000 miles
Root cause
The AdBlue dosing system can suffer from pump failure or injector clogging due to urea crystallisation. Fault codes P203B31 (AdBlue fill level sensor) and P206B31 (AdBlue concentration sensor) can trigger countdown warnings and eventual engine start prevention.

Repair options

Repair tierDealer priceSpecialist price
Minor - AdBlue system diagnostics£150-£300£100-£200
Intermediate - AdBlue injector/pump replacement£600-£1,200£350-£800
Full Replacement - Rebuilt OM656 engine£7,500-£11,000£2,800-£4,900
Our recommendation
AdBlue faults are primarily emission-system issues, not core mechanical failures. If your S 350d or S 400d displays a countdown warning, have the SCR system inspected immediately. Specialist AdBlue injector or pump replacement at £350-£800 is significantly cheaper than dealer rates. On an S-Class worth £25,000-£60,000, preventative attention is always the better investment.
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Don't Let EGR Clogging, Vacuum Leaks, or AdBlue Issues Write Off Your Mercedes S 350d or 400d

Whether you're facing OM656 EGR system issues on your S 350d, vacuum pump coolant leaks on your S 400d, or camshaft wear, EnginesMarket makes it simple to compare quotes from trusted UK Mercedes diesel specialists. Every rebuilt engine in our network carries upgraded components addressing the original design weakness - new EGR components, updated coolant pump parts, and fresh valve train components - plus a minimum 12-month unlimited mileage warranty with nationwide delivery and supply & fit options. Don't pay main dealer premiums. Compare prices from independent Mercedes specialists who know these OM656 inline-six diesels inside out.

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FAQ

Mercedes Benz S Class 350d/400d Engine ReplacementFrequently Asked Questions

Everything Mercedes Benz S Class 350d/400d owners usually ask before choosing a rebuilt, reconditioned or used replacement engine.

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Compare Mercedes S Class 350d/400d Engine Prices Today

Comparing quotes for your Mercedes S 350d or S 400d is completely free, with no obligation and no pressure. Enter your registration above to get tailored quotes from UK Mercedes diesel specialists who understand the OM656 inline-six engine inside out. Whether you need a used engine, a reconditioned unit, or a full rebuilt engine with upgraded EGR components, vacuum system parts, and valve train components, we'll connect you with the right specialist for your exact S-Class - with a minimum 12-month warranty on every engine supplied.

200+200+ OM656 inline-six diesel engines supplied through our network
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