Mercedes Benz S Class 350d/400d Engine ReplacementCompare 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.
Supply & Fit Available
12-Month Warranty
Nationwide Delivery
100+ Suppliers
From £2,400 to £4,900
Used from £1,500 · Reconditioned from £2,400 · Common codes: OM656.929, OM656.932, OM656.964
From £1,500
Tested, bare/short engine
From £2,400
Rod bearings addressed, tested
From quote
Upgraded bearings, full teardown, revised valvetrain
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3 Simple Steps3 Simple Stepsto Finding the Best Mercedes Benz S Class 350d/400d Replacement Engineto Finding the Best Mercedes Benz S Class 350d/400d Replacement Engine
Most replacements completed within 3-5 days.
Variant HistoryMercedes 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.
Variant HistoryMercedes 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
Fuel TypeDiesel (inline-six turbodiesel with AdBlue SCR)
Engine Size2.9L (2,925cc)
Power Output Range286-340hp across the variant's historyEngine History Timeline
OM656.929 3.0L inline-six diesel, 286hp @ 3,400-4,600rpm (S 350d, W222 facelift)
OM656.929 3.0L inline-six diesel, 340hp @ 3,600-4,200rpm (S 400d, W222 facelift)
OM656.929 3.0L inline-six diesel, 340hp (S 400d 4MATIC)
W223 generation launch; OM656 continues with refinements
2023 - OM656.929 2.9L inline-six diesel, 282hp (S 350d, W223)
present - OM656 continues as the flagship diesel for S-Class
Key Specs Snapshot
Fuel TypeDiesel (inline-six turbodiesel with AdBlue SCR)Engine Size2.9L (2,925cc)Power Output Range286-340hp across the variant's historyYears Produced2017-presentEvery 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.
Mercedes Benz S Class 350d/400d Engine Codes - Specifications & Replacement CostMercedes Benz S Class 350d/400d Engine Codes - Specifications & Replacement Cost
2.9L Inline-Six Diesel (2017-present)
Power Output286 hp (210 kW) @ 3,400-4,600 rpm (S 350d); 340 hp (250 kW) @ 3,600-4,200 rpm (S 400d)Torque600 Nm (S 350d); 700 Nm (S 400d)Years Fitted2017-presentDisplacement2,925 ccCylindersInline 6, DOHC, 24-valve, twin-turbochargedEngine CodeOM 656.929
Compatible Trim Levels
- S 350d
- S 350d 4MATIC
- S 400d
- S 400d 4MATIC (W222 facelift, W223)
Find your exact engine code, specs and UK replacement cost guide.
Used
Tested, ready-to-fit engine
£1,500-£2,500
Reconditioned
Addressed known issues, tested
£2,400-£3,600
Rebuilt
Full teardown, upgraded internals
£2,800-£4,900
Common Failure
EGR system clogging and cooler issues - the OM656 features both low-pressure and high-pressure EGR systems, which can become clogged with carbon deposits over time. Vacuum pump issues have been reported in the 25,000-40,000 mile range. Camshaft and rocker wear can occur around 100,000 miles. Coolant pump vacuum circuit leaks affecting 800,000 OM654/OM656 vehicles have been subject to safety recalls.
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.
The Modern Diesel's Emissions Challenge
Affected Vehicles
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.
Why It Happens
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.
Dealer (parts + labour)
£400-£700
Specialist (parts + labour)
£250-£450
What it involves
Remove and clean EGR valve and cooler passages, system recalibration
Suitability
If caught early with mild symptoms
1 - 2 daysDealer (parts + labour)
£1,200-£2,000
Specialist (parts + labour)
£750-£1,200
What it involves
Replace EGR cooler unit, clean intake passages, reset adaptations
Suitability
If cleaning does not resolve the issue
1 - 2 daysDealer (parts + labour)
£7,500-£11,000
Specialist (parts + labour)
£2,800-£4,900
What it involves
Complete engine swap with new EGR components, 12-month warranty
Suitability
If EGR failure has caused engine damage
1 - 2 daysAll prices are indicative UK market ranges based on current UK specialist supplier data.| Repair Tier | Dealer (Parts + Labour) | Specialist (Parts + Labour) | What It Involves | Suitability | Estimated Time |
|---|---|---|---|---|---|
 Minor - EGR system cleaning Minor | £400-£700 | £250-£450 | Remove and clean EGR valve and cooler passages, system recalibration | If caught early with mild symptoms | 1 - 2 days |
 Intermediate - EGR cooler replacement Best Value | £1,200-£2,000 | £750-£1,200 | Replace EGR cooler unit, clean intake passages, reset adaptations | If cleaning does not resolve the issue | 1 - 2 days |
Full Replacement - Rebuilt OM656 engine Best Value | £7,500-£11,000 | £2,800-£4,900 | Complete engine swap with new EGR components, 12-month warranty | If EGR failure has caused engine damage | 1 - 2 days |
Vehicle Value Check
Vehicle Value Check
A typical 2017-present S 350d or S 400d is worth approximately £25,000-£60,000 depending on mileage and condition. The S-Class flagship retains strong residual values - replacement cost is typically small relative to vehicle value.
Our Recommendation
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.
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.
Compare rebuilt OM656 Mercedes S 350d/400d engine prices
12-Month Minimum
UK Specialists Vetted
Best Price GuidanceImportant Information
Important Information
"All prices are indicative UK market ranges based on historical quote data and repair requests from our network of engine specialists. Dealer pricing is estimated based on published main dealer labour rates and genuine OEM parts pricing. Specialist pricing reflects actual quote data from EnginesMarket's UK network. Actual costs vary depending on damage severity, vehicle condition, variant, and workshop location within the UK. Use our free comparison tool to get tailored quotes for your specific Mercedes S Class 350d or 400d. Every engine supplied through EnginesMarket carries a minimum 12-month unlimited mileage warranty - always confirm exact terms with your chosen supplier."
Mercedes Benz S Class 350d/400d Engine ReplacementFrequently Asked Questions
Question 1How much does a Mercedes S 350d or S 400d engine replacement cost in the UK-
Short summary: A rebuilt OM656 engine for the S 350d or S 400d costs £2,800-£4,900 supply-only. Fitting adds £1,200-£2,500 depending on complexity - the inline-six requires specialist knowledge.
- S 350d / S 400d (OM656.929): rebuilt unit £2,800-£4,900; used engine from £1,500
- OM656 EGR cooler replacement: £750-£1,200 at specialist
- OM656 camshaft/rocker replacement: £1,200-£2,000 at specialist
Question 2What are the signs of EGR failure on a Mercedes S 350d or S 400d-
Short summary: Reduced performance, increased fuel consumption, rough idling and dashboard warning lights are the classic warning signs on the OM656 engine. The EGR valve and cooler can become clogged with soot over time.
- Reduced power and poor throttle response
- Increased fuel consumption
- Rough idling - one owner reported very rough idle with white smoke
- Check engine light with emissions-related fault codes
- If caught early, cleaning costs £250-£450 at specialist
Question 3Is the Mercedes S 350d or S 400d engine (OM656) reliable-
Short summary: The OM656 is considered "pretty reliable" by specialists. However, it has known issues: vacuum pump problems in the 25,000-40,000 mile range, camshaft and rocker wear around 100,000 miles, and EGR system clogging. With these addressed, the engine is proving durable.
- OM656 is fundamentally robust with excellent power delivery
- Vacuum pump issues affect early models - check recall status
- Camshaft and rocker wear can occur at higher mileage
- Well-maintained OM656 engines exceed 100,000 miles
Question 4How long do Mercedes S 350d and S 400d engines last-
Short summary: The OM656 is too new for definitive long-term data, but with regular maintenance and proactive attention to known issues, 150,000+ miles is achievable. Some early failures have been reported, but overall reliability is improving.
- OM656 (S 350d/400d): too early for definitive data - EGR and vacuum issues from 25k-100k miles
- Service interval is 10,000 miles or 12 months - use Mercedes-spec low-SAPS oil (MB 229.52)
- Late oil changes accelerate camshaft and rocker wear
Question 5Is it worth replacing the engine in my Mercedes S 350d or S 400d-
Short summary: Yes, if the S-Class is otherwise in good condition with no rust or major electrical faults. A rebuilt OM656 costs £2,800-£4,900 - far less than replacing a £25,000-£60,000 vehicle. The S-Class's strong residual values make engine replacement financially justifiable.
- S 350d/400d values: £25,000-£60,000 depending on year, mileage and condition
- Rebuilt engine with upgraded components fixes original weak points
- Supply & fit typically £4,000-£7,400 depending on engine code
- New engine includes 12-month unlimited mileage warranty
Question 6What oil does my Mercedes S 350d or S 400d need-
Short summary: Mercedes-Benz specifies low-SAPS oil meeting MB 229.52 - typically 5W-30. The OM656 inline-six requires approximately 8.5 litres.
- OM656 (S 350d/400d): MB 229.52 5W-30 low-SAPS - protects DPF and AdBlue systems
- Never use non-approved oil - can cause camshaft and rocker wear
- Change every 10,000 miles or 12 months - stretched intervals cause valve train wear
Why Choose UsCompare Mercedes S Class 350d/400dEngine PricesToday
200+ OM656 inline-six diesel engines supplied through our network
12-Month Warranty on every rebuilt S 350d and S 400d engine
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supply & fit available for OM656
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.
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