How much does a Mercedes‑Benz S350d / S400d engine replacement cost in the UK?

A reconditioned OM656 engine typically costs £3,500–£6,500 supply only for the S350d, and £4,000–£8,000 for the S400d. Fitting adds £1,000–£1,600. Camshaft and rocker wear repairs can cost £1,500–£3,500 if caught early. Use our calculator for a figure specific to your car.

What are the common problems with the Mercedes‑Benz S350d / S400d?

Turbo actuator vacuum hose collapse (intermittent power loss, gearbox judder), camshaft rocker wear (popping noise from intake), AdBlue system faults (NOx sensors, pump), DPF clogging, vacuum pump oil leaks (early cars), and fuel system contamination. The turbo hose issue is particularly common on the S400d.

Is it worth replacing a Mercedes‑Benz S350d / S400d engine, or should I scrap the car?

Depends on vehicle value. Against a well‑kept W223 S400d worth £40,000–£70,000, a £4,000–£8,000 recon is a manageable 6–20% of the vehicle's value — worth it. For a W222 S350d worth £15,000–£25,000, a £4,500 recon is more marginal but often worthwhile given the S-Class's status and retained values.

How do I find my Mercedes‑Benz S350d / S400d engine code?

Listed on the VIN plate and vehicle documentation. The S350d and S400d both use the OM656 2.9-litre inline-six diesel engine. Tell us your registration date and we'll confirm it for free.

Can I repair the OM656's turbo actuator hose, or does it need a new engine?

Yes — turbo actuator hose replacement is a straightforward repair costing £300–£800. Mercedes is investigating a factory fix for the collapsing hose issue. This does not typically require engine replacement. At the first sign of power loss or gearbox judder, book an inspection immediately.

Mercedes-Benz S350d / S400d2017–presentOM656 2.9 Straight-Six Diesel

Mercedes-Benz S350d / S400d Engine Replacement — Compare Prices & Get Quotes

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Mercedes S350d / S400d Engines — from £3,000 to £8,000Code: OM656

Mercedes S350d / S400d Engines — from £3,000 to £8,000· Code: OM656

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The S350d / S400d THROUGH TIME — Era Map

Four generations. Four eras. One guide to reliability, costs and what really matters.

Era Note

First-generation OM656 in the S-Class — robust and refined, but watch for turbo actuator vacuum hose collapse and cam rocker wear.

GenerationYearsEngine CodeReliabilityRecon CostEra Note

W222 (S350d) BMW 320d W222 (S350d)

2017–2020OM656 (2.9 straight-six, 286 bhp)★★★★☆ [NEWLY ASSIGNED — see Data Note]£3,500–£6,500 [MB‑QUOTE]First-generation OM656 in the S-Class — robust and refined, but watch for turbo actuator vacuum hose collapse and cam rocker wear.

W223 (S350d) BMW 320d W223 (S350d)

2020–presentOM656 (2.9 straight-six, 286 bhp)★★★★☆ [NEWLY ASSIGNED — see Data Note]£3,500–£7,000 [MB‑QUOTE]The modern S350d — improved OM656 with 48V mild-hybrid; AdBlue system faults and valvetrain wear are the main watchpoints.

W223 (S400d) BMW 320d W223 (S400d)

2020–presentOM656 (2.9 straight-six, 330 bhp)★★★★☆ [NEWLY ASSIGNED — see Data Note]£4,000–£8,000 [MB‑QUOTE]The higher-output OM656 — 330 bhp, available with 4MATIC; same engine architecture with higher boost and more emissions complexity.

Why use this era map?

Every cost guide, reliability rating and engine code on this page is aligned to the data above. This keeps everything consistent and trustworthy.

Real UK Cost Data

Verified Reliability

Expert Insights

Updated for 2025

Mercedes-Benz S350d / S400d Engine Replacement Costs

Supply Only£3,500–£6,500

Fitted (Indie)£4,500–£7,800

Warranty12–24 months

Best ForMost owners — best cost/warranty balance

Engine TypeSupply OnlyFitted (Indie)WarrantyBest For

Used Engine£3,000–£5,000£3,800–£6,2003–6 monthsBudget priority; vehicle under £10,000

Reconditioned Engine£3,500–£6,500£4,500–£7,80012–24 monthsMost owners — best cost/warranty balance

Rebuilt Engine£5,000–£8,000£6,000–£9,30012–24 monthsLong‑term ownership; 3+ year horizon

Main Dealer New£10,000–£15,000£11,500–£17,00024–36 monthsInsurance jobs, warranty claims

Labour estimate: Fitting an S350d / S400d engine typically takes 12–16 hours at £65–£95/hr (independent) or £110–£160/hr (London/SE dealer). Camshaft and rocker wear repairs require cylinder head work and are expensive.

Common Problems

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Mercedes-Benz S350d / S400d — Repair, Buy, or Replace?

Every 320d has a story. Make the right call with clear data, real-world costs, and honest guidance from UK specialists.

Mercedes-Benz S350d / S400d — Repair, Buy, or Replace?

Every 320d has a story. Make the right call with clear data, real-world costs, and honest guidance from UK specialists.

Can It Be Repaired?

ProblemRepairable?Typical CostWhen It Makes SenseTurbo actuator vacuum hose collapse (OM656)✅ Yes£300–£800Replace with updated hose; common on S400dCamshaft rocker arm wear (OM656)✅ Yes£1,500–£3,500If caught early; don't drive with intake poppingAdBlue system failure (OM656)✅ Yes£500–£1,500Replace NOx sensors, pump or injector; don't let countdown expireVacuum pump oil leak (OM656 — early)✅ Yes£500–£1,200If it hasn't happened by 40k miles, it won'tDPF clogging (OM656)✅ Yes£400–£1,200Preventative — regular motorway runs are the best preventionFuel system contamination (OM656)✅ Yes£500–£2,000Use high-quality diesel and regular filter changes

Buying This S350d / S400d? Check These First

Buy If

Full Mercedes service history with documented oil changes
Post-2020 W223 — generally more refined than the W222
No AdBlue system issues or evidence of NOx sensor replacement
Vacuum pump oil leak not present (if it hasn't happened by 40k, it won't)
No intake popping noise — indicates rocker wear

Avoid If

Intermittent power loss or gearbox judder — potential turbo actuator hose issue
Intake popping noise — camshaft rocker wear warning
AdBlue countdown active — potential system failure
Poor service history — the OM656 is sensitive to fuel and oil quality
High-mileage example (100k+) without documented valvetrain inspection

Closing Verdict

The S350d / S400d's OM656 is Mercedes' most refined diesel engine — a smooth, powerful straight-six that delivers effortless performance in the flagship S-Class. It's a significant step forward from the OM642 V6 in refinement and efficiency. However, it has known issues: turbo actuator vacuum hose collapse (particularly on the S400d), camshaft rocker wear around 100,000 miles, and AdBlue system faults. Buy on documented history, and if you're looking at a high-mileage example, demand proof of valvetrain inspection. If you're replacing, a reconditioned unit with a 12‑month warranty is the sweet spot.

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Buying Checklist

Before you buy a used Mercedes‑Benz S350d / S400d (or before you commit to an engine replacement), check these:

Turbo Health — Test acceleration; any intermittent power loss or judder? Turbo actuator vacuum hose collapse risk

Intake Noise — Listen for popping noise from the intake — camshaft rocker wear warning

Cold Start — Any rattling or ticking? Valvetrain wear indicator

Service History — Including which engine era (OM656 is the only S350d/S400d engine — verify this before purchase) and fuel filter change history

AdBlue System — Any AdBlue warnings? Countdown active? NOx sensor or pump failure

Oil Leaks — Check for oil spots under the car — vacuum pump oil leak (early cars)

DPF Status — Any DPF warning lights or regeneration issues?

Engine Warning Light — Have codes been read? Any unresolved turbo, AdBlue, or emissions issues?

Buy smart. Drive with confidence.

A few checks today can save thousands tomorrow. Use this checklist to avoid the most common 320d pitfalls.

Buy smart. Drive with confidence.

A few checks today can save thousands tomorrow. Use this checklist to avoid the most common 320d pitfalls.

Engine Codes

OM656 (S350d)

Years2017–present

Power286 bhp / 600 Nm

2025 Enquiries— (pattern data)

Avg. Recon Cost£3,500–£7,000 [MB‑QUOTE]

Engine CodeYearsPower2025 EnquiriesAvg. Recon Cost

OM656 (S350d)2017–present286 bhp / 600 Nm— (pattern data)£3,500–£7,000 [MB‑QUOTE]

OM656 (S400d)2020–present330 bhp / 700 Nm— (pattern data)£4,000–£8,000 [MB‑QUOTE]

Technical Specs

OM656

Displacement2,925cc

Cylinders6‑cylinder inline

BlockClosed-deck aluminium with Nanoslide coating

PistonsSteel

Compression Ratio15.5:1

Fuel SystemCommon‑rail direct injection (Bosch CP4)

TurbochargerSingle variable geometry turbo (VGT) with electric actuator

EmissionsEuro 6d with AdBlue (SCR) and EGR

Market Intelligence

Most Common Failure

Turbo actuator vacuum hose collapse (S400d), camshaft rocker wear (~100k miles), and AdBlue system faults — approximately 55% of OM656 enquiries

Average Enquiry Mileage

52,000

miles

[PATTERN DATA]

Average Replacement Cost

£5,000

reconditioned, supply only

[MB‑QUOTE]

Most Requested Engine Code

OM656

[MB‑VERIFIED]

Peak Enquiry Season

Winter — cold starts expose turbo and valvetrain issues

[PATTERN DATA]

Regional Demand

London25%

South East22%

South West14%

PATTERN DATA

Regional Demand Breakdown

25%London

22%South East

Frequently Asked Questions

All prices are indicative UK market ranges based on historical quote data from our network of engine specialists. Actual costs vary by variant, engine code, damage severity and workshop location. Use our free comparison tool to get tailored quotes. Every engine supplied through EnginesMarket carries a minimum 12‑month unlimited‑mileage warranty — always confirm terms with your chosen supplier.

Find the best Mercedes‑Benz S350d / S400d engine replacement UK drivers rely on. Compare prices from trusted specialists on EnginesMarket today and keep your flagship on the road.

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100% Free

No hidden fees

No Obligation

Get quotes with zero pressure

UK Specialists

20+ vetted experts

12-Month Warranty

As standard

Nationwide Delivery

Supply & fit available