Is the Mercedes CLS 350d or 400d engine reliable?

The OM656 is generally considered a reliable engine — a modern, efficient straight-six diesel that replaced the OM642 V6. However, it has known issues: Camtronic variable valve timing problems (camshaft rocker wear causing popping intake noises), Bosch CP4 fuel system sensitivity (poor fuel damages piezo injectors), turbocharger issues (oil deposits, actuator hose failure), and AdBlue/DPF system faults. Regular maintenance and quality fuel are essential.

What are the common problems with the Mercedes CLS 350d and 400d?

The most frequently reported issues are Camtronic camshaft rocker wear (popping intake noise, rough idle, engine warning light), fuel system issues (Bosch CP4 injectors sensitive to fuel quality), turbocharger faults (oil deposits, actuator issues, boost lag), vacuum oil pump issues (early cars, 25–40k miles), and AdBlue/DPF system problems.

Is it worth replacing a Mercedes CLS diesel engine, or should I scrap the car?

Depends on vehicle value. Against a well-kept £35,000 CLS 400d, a £7,000 recon is 20% — worth it. Against a tired £15,000 example, the same recon approaches our 70% Mercedes threshold — get a scrap valuation first.

How do I find my Mercedes CLS diesel engine code?

Listed on the VIN plate and vehicle documentation. CLS 350d: OM656 D29 R SCR. CLS 400d: OM656 D29 SCR. Both are 2.9-litre straight-six twin-turbo diesel engines. The badge on the tailgate is the easiest tell — 'CLS 350 d' or 'CLS 400 d'.

Can I repair the camshaft rocker wear on the OM656, or does it need full engine replacement?

Yes — if caught early. Camshaft rocker wear is a known issue that will show up as a popping sound in the intake. Scoring of the cam lobes only happens when the car is driven with the inlet popping for thousands of miles. Repair costs typically range from £800–£2,500. If ignored, the issue can lead to cam lobe scoring, timing failure, and catastrophic engine damage. At the first sign of popping intake noises, book an inspection immediately. Regular oil changes are the single most effective preventative measure.

Mercedes CLS 350d / 400d2017–present2.9L Straight-Six Twin-Turbo Diesel (OM656)

Mercedes CLS 350d & 400d Engine Replacement — Compare Prices & Get Quotes

Name: Mercedes CLS Diesel UK Engine Enquiry Data 2025
Creator: MercedesEngine.uk

Get quotes today from trusted UK Mercedes CLS specialists. Compare reconditioned, rebuilt and used OM656 2.9-litre straight-six twin-turbo diesel engines — in 350d and 400d tune — supply & fit available nationwide.

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Supply & Fit Available

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Mercedes CLS 350d/400d Engines — from £3,500 to £9,500Code: OM656

Mercedes CLS 350d/400d Engines — from £3,500 to £9,500· Code: OM656

Instant QuotesNo Obligation100% FreeReconditionedRebuiltUsedSupply & Fit AvailableUK-Wide Network

The CLS 350d / 400d THROUGH TIME — Era Map

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

Era Note

One engine family throughout this badge's history — no era ambiguity. 350d: 286 hp / 600 Nm; 400d: 340 hp / 700 Nm.

GenerationYearsEngine CodeReliabilityRecon CostEra Note

C257 CLS 350d / 400d BMW 320d C257 CLS 350d / 400d

2017–presentOM656 (2.9L Straight-Six Twin-Turbo Diesel)★★★★☆ [NEWLY ASSIGNED — see Data Note]£5,500–£8,500 [PLACEHOLDER — confirm vs cost sheet]One engine family throughout this badge's history — no era ambiguity. 350d: 286 hp / 600 Nm; 400d: 340 hp / 700 Nm.

Why use this era map?

This table is the single source every other section on this page matches — Sections 2, 4 and 6 reference it rather than restating conflicting figures.

Real UK Cost Data

Verified Reliability

Expert Insights

Updated for 2025

Mercedes CLS 350d / 400d Engine Replacement Costs

Supply Only£5,500–£8,500

Fitted (Indie)£6,800–£10,000

Warranty12–24 months

Best ForMost owners — best cost/warranty balance

Engine TypeSupply OnlyFitted (Indie)WarrantyBest For

Used Engine£3,500–£5,500£4,800–£7,2001–6 monthsBudget priority; lower-mileage examples

Reconditioned Engine£5,500–£8,500£6,800–£10,00012–24 monthsMost owners — best cost/warranty balance

Rebuilt Engine£6,500–£9,500£7,800–£11,00012–24 monthsLong-term ownership; 3+ year horizon

Main Dealer New£10,000–£14,000+£11,500–£16,000+24–36 monthsInsurance jobs, warranty claims

Figures above reflect the OM656 2.9-litre straight-six diesel engine. A full engine replacement can run anywhere between £1,500 and £5,000 on average, with Mercedes engine replacements typically around £3,500 or more. Used OM656 engines are available from approximately £3,500–£5,500.

Labour estimate: Fitting a CLS OM656 engine typically takes 14–18 hours at £75–£110/hr (independent) or £130–£180/hr (London/SE dealer). The straight-six layout and engine bay packaging make this a moderate-to-complex job.

→ Compare CLS diesel engine prices from UK specialists

Common Problems

1. Camtronic Variable Valve Timing Issues — Camshaft Rocker Wear

Symptoms: Popping noise from the intake manifold, rough idle, engine warning light, loss of power, metallic ticking.

Typical mileage: 60,000–100,000 miles (sometimes less)

Repair cost: £800–£2,500 (camshaft rocker and actuator replacement)【PLACEHOLDER — confirm vs cost sheet】

Replacement cost: £5,500–£9,500 (if left untreated leads to cam lobe scoring and timing failure)

🟠 Check - book inspection urgently

Our recommendation:

Cam rocker wear is a known issue on the OM656. It will show up as a popping sound in the intake. Scoring of the cam lobes only happens when the owner has driven the car with the inlet popping away for a few thousand miles. If you notice unusual noises from the engine, have the Camtronic system inspected immediately. Regular oil changes are critical for this engine.

Read the full Camtronic failure guide

1. Camtronic Variable Valve Timing Issues — Camshaft Rocker Wear

Symptoms: Popping noise from the intake manifold, rough idle, engine warning light, loss of power, metallic ticking.

Typical mileage: 60,000–100,000 miles (sometimes less)

Repair cost: £800–£2,500 (camshaft rocker and actuator replacement)【PLACEHOLDER — confirm vs cost sheet】

Replacement cost: £5,500–£9,500 (if left untreated leads to cam lobe scoring and timing failure)

🟠 Check - book inspection urgently

Our recommendation:

Read the full Camtronic failure guide

2. Fuel System Sensitivity — Bosch CP4 Injector Issues

Symptoms: Rough running, misfires, poor acceleration, engine warning light, increased fuel consumption.

Typical mileage: 40,000–80,000 miles

Repair cost: £600–£2,000 (injector replacement — up to six piezo injectors)【PLACEHOLDER — confirm vs cost sheet】

Replacement cost: £5,500–£9,500 (if fuel system damage leads to engine failure)

🟡 Monitor - book inspection soon

Our recommendation:

The OM656 uses a Bosch CP4 common-rail system with sensitive piezo injectors. It does not tolerate poor-quality diesel or contamination. Even minor impurities in the fuel tank can damage the injectors, while a clogged fuel filter disrupts combustion and leads to power loss. Regular fuel filter changes and the use of high-quality diesel can significantly extend engine life. Bad fuel can also cause soot buildup on valves and intake channels.

Read the full fuel injector failure guide

3. Turbocharger & AdBlue/DPF System Issues

Symptoms: Loss of power, unusual noises from the engine bay (whistling, grinding), turbo lag, DPF warning light, AdBlue system errors, engine management light.

Typical mileage: 40,000–80,000 miles

Repair cost: £800–£2,500 (turbocharger repair or replacement; AdBlue/DPF system repairs)【PLACEHOLDER — confirm vs cost sheet】

Replacement cost: £5,500–£9,500 (if turbo failure or emissions system damage leads to engine damage)

🟠 Check - book inspection urgently

Our recommendation:

The OM656's BorgWarner R2S turbocharger is a sensitive component. Oil deposits, worn bearings, or a contaminated charge air system can lead to power loss and unusual noises. A known turbo actuator issue involves a hose that gets too hot and collapses, causing boost faults. The AdBlue and DPF systems are also sensitive to poor fuel quality and short journeys. Regular regeneration drives and high-quality AdBlue minimise risks.

Read the full turbocharger failure guide

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CLS 350d / 400d — 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.

CLS 350d / 400d — 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 SenseCamshaft rocker wear (Camtronic)✅ Yes£800–£2,500Catch it early before cam lobe scoring occursFuel injector issues (CP4 system)✅ Yes£600–£2,000Use quality fuel and regular filter changesTurbocharger faults✅ Yes£800–£2,500Early detection prevents engine damageAdBlue/DPF system faults✅ Yes£500–£2,000Regular regeneration and quality AdBlueTiming chain stretch✅ Yes£1,500–£3,000Preventative — catches the fault before catastrophic failureMajor internal engine failure❌ Usually No£5,500–£9,500Typically requires full engine replacement or complete rebuild

Buying This CLS Diesel? Check These First

Buy If

Full Mercedes service history with evidence of regular oil changes using MB 229.5 or 229.51 spec oil
Camtronic system inspected or serviced (no popping intake noises)
Fuel system regularly maintained (quality fuel, regular filter changes)
Turbocharger and AdBlue/DPF system inspected
Early OM656 vacuum oil pump issue resolved (if applicable)

Avoid If

Popping noise from the intake (cam rocker wear warning)
Rough running or misfires (injector issues)
Loss of power or turbo lag (turbo or actuator issues)
Poor service history — the OM656 is a sophisticated engine that demands good maintenance
Engine warning light with no diagnosis

Closing Verdict

The CLS 350d and 400d are powered by Mercedes' modern OM656 straight-six diesel — a sophisticated and refined engine that delivers exceptional performance and efficiency. The 350d offers 286 hp and 600 Nm, while the 400d delivers 340 hp and 700 Nm. Both are generally reliable engines, but they have known weak points: Camtronic valve timing issues, Bosch CP4 fuel system sensitivity, turbocharger concerns, and AdBlue/DPF system faults. These are well-understood and manageable with proper maintenance. Buy on documented history, not on faith. A well-maintained CLS diesel is a £25,000–£50,000 premium four-door coupe; a neglected one is a £7,000 engine replacement waiting to happen.

Don't guess.Get the right engine at the right price.

→ Compare CLS diesel engine prices before you buy

Don't Guess. Compare.

Get real quotes from vetted UK specialists and make the right call for your 320d.

→ Compare CLS diesel engine prices before you buy

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Buying Checklist

Before you buy a used Mercedes CLS 350d or 400d (or before you commit to an engine replacement), check these:

Cold Start — Listen for popping noises from the intake (cam rocker wear warning)

Service History — including the OM656 era (2017–present) and confirmed oil-change intervals using MB 229.5/229.51 spec oil

Camtronic Check — Any history of camshaft rocker or actuator issues?

Fuel System Check — Evidence of regular fuel filter changes and quality fuel use

Turbo Check — Any loss of power, turbo lag, or unusual noises? (turbo or actuator warning)

DPF/AdBlue Check — Any DPF or AdBlue warning lights or regeneration issues?

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

MOT History — Check for emissions-related failures

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

This table's rows match Section 1B's Era Map exactly.

Engine CodeYearsPower2025 EnquiriesAvg. Recon Cost

Technical Specs

OM656 — both variants

Displacement2,927 cc

CylindersStraight-6, 4 valves per cylinder

Bore x Stroke82.0 mm × 92.4 mm

Compression Ratio15.5:1

Fuel SystemCommon-rail direct injection (Bosch CP4 with piezo injectors)

TurbochargerBorgWarner R2S sequential twin turbochargers

ValvetrainDOHC with Camtronic variable valve timing

BlockAluminium with plasma-sprayed cast iron Nanoslide coating

Transmission9G-Tronic 9-speed automatic with 4MATIC

→ Compare CLS diesel engine prices for your Mercedes

Get quotes from trusted UK specialists in minutes.

Market Intelligence

Most Common Failure

Camshaft rocker wear (Camtronic system) — popping intake noise is the telltale symptom

Average Enquiry Mileage

40,000

–70,000 miles

[PATTERN DATA]

Average Replacement Cost

£5,500

–£8,500 reconditioned, supply only

[PLACEHOLDER — confirm vs cost sheet]

Most Requested Engine Code

OM656 (350d / 400d)

[PATTERN DATA]

Peak Enquiry Season

Winter — cold starts expose Camtronic and turbo issues【PATTERN DATA】

Regional Demand

London22%

South East18%

Midlands14%

PATTERN DATA

Regional Demand Breakdown

22%London

18%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.

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Find the best Mercedes CLS 350d or 400d engine replacement UK drivers rely on. Compare prices from trusted specialists on EnginesMarket today and get your CLS back on the road with confidence.

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No hidden fees

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UK Specialists

20+ vetted experts

12-Month Warranty

As standard

Nationwide Delivery

Supply & fit available