How much does a Mercedes CLS 450 / 500 engine replacement cost in the UK?

A reconditioned CLS 450/500 (M256) engine typically costs £4,500–£7,500 supply only. Fitting adds £2,000–£3,500 depending on location and workshop rates. Main dealer new engines start from £9,000–£15,000+ supply only. Use our calculator for a figure specific to your car.

Is the Mercedes CLS 450 / 500 engine reliable?

The M256 is considered 'robust and reliable' when serviced regularly. It marked Mercedes' return to the inline-six layout, combining smoothness with modern efficiency. However, it has documented issues: coolant pump and 48V system failures (subject to recall), timing chain noise on cold start, wiring harness faults, and carbon buildup. With proper maintenance, these engines can be reliable, but the coolant pump issue is a genuine concern.

What are the common problems with the Mercedes CLS 450 / 500?

The main known issues are: (1) Coolant pump / 48V system failure — electric line chafing on the engine fan; subject to recall, (2) Timing chain noise on cold start — rattling for a few seconds on startup, (3) Wiring harness and electrical faults — causing misfires and ECU issues, and (4) Carbon buildup on intake valves — causing rough idle and misfires.

Is it worth replacing a Mercedes CLS 450 / 500 engine, or should I scrap the car?

Depends on vehicle value and condition. Against a well-kept £18,000–£30,000 CLS, a £5,000–£7,500 recon is 20–40% of the car's value. For a clean example with full service history, replacement is often worth it given the CLS's strong retained values and the M256's reputation when properly maintained. However, if the car has other issues (air suspension, electrical faults, accident damage), evaluate total repair costs before committing.

How do I find my Mercedes CLS 450 / 500 engine code?

The CLS 450 and 500 use the M256 engine code — a 3.0-litre turbocharged inline-six with 48V mild hybrid. The CLS 450 produces 362 hp (270 kW) and 369 lb-ft (500 Nm); the CLS 500 produces 381–388 hp and 384 lb-ft in select markets. The engine code is stamped on the cylinder block. Tell us your registration and we'll confirm it for free.

Mercedes CLS 450 / 5002018–20231 Engine Generation

Mercedes CLS 450 / 500 Engine Replacement — Compare Prices & Get Quotes

Name: Mercedes CLS 450/500 UK Engine Enquiry Data 2025
Creator: MercedesEngine.uk

Get quotes today from trusted UK Mercedes CLS specialists. The 3.0-litre M256 inline-six turbo with 48V mild hybrid powers the C257 CLS — compare reconditioned, rebuilt and used engines with supply & fit available nationwide.

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Mercedes CLS 450 / 500 Engines — from £2,500 to £8,500Engine code: M256

Mercedes CLS 450 / 500 Engines — from £2,500 to £8,500· Engine code: M256

Instant QuotesNo Obligation100% FreeReconditionedRebuiltUsedSupply & Fit AvailableUK-Wide Network

Mercedes CLS 450 / 500 — Which Engine Does Your Car Actually Have?

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

Era Note

3.0L inline-six turbo with 48V EQ Boost — 362 hp, 369 lb-ft. Coolant pump and wiring harness issues documented

GenerationYearsEngine CodeReliabilityRecon CostEra Note

C257 (CLS 450) BMW 320d C257 (CLS 450)

2018–2023M256 (3.0L I6 turbo)★★★☆☆ [NEWLY ASSIGNED — see Data Note]£4,500–£7,500 [MB-QUOTE]3.0L inline-six turbo with 48V EQ Boost — 362 hp, 369 lb-ft. Coolant pump and wiring harness issues documented

C257 (CLS 500) BMW 320d C257 (CLS 500)

2018–2023M256 (3.0L I6 turbo)★★★☆☆ [NEWLY ASSIGNED — see Data Note]£4,500–£7,500 [MB-QUOTE]3.0L inline-six turbo with 48V EQ Boost — 381–388 hp. Higher-output version of the M256

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 CLS 450 / 500 Engine Replacement Costs

Supply Only£4,500–£7,500

Fitted (Indie)£6,500–£9,500

Warranty12–24 months

Best ForMost owners — best cost/warranty balance

Engine TypeSupply OnlyFitted (Indie)WarrantyBest For

Used Engine£2,500–£5,000£3,500–£6,5001–6 monthsBudget priority; lower-mileage donor

Reconditioned Engine£4,500–£7,500£6,500–£9,50012–24 monthsMost owners — best cost/warranty balance

Rebuilt Engine£5,500–£8,500£7,500–£10,50012–24 monthsLong-term ownership; 3+ year horizon

Main Dealer New£9,000–£15,000+£11,000–£17,000+24–36 monthsInsurance jobs, warranty claims

Labour estimate: Fitting a CLS 450/500 engine typically takes 14–18 hours at £65–£95/hr (independent) or £110–£160/hr (London/SE dealer). The 48V mild-hybrid system and complex wiring add labour time.

→ Compare CLS 450 / 500 engine prices from UK specialists

Common Problems

1. Coolant Pump & 48V System Failure

Symptoms: Erratic engine cooling, warning lights, coolant loss, overheating, electric line chafing on the engine fan

Typical mileage: Any mileage — a recall affects certain CLS vehicles with the M256 engine

Repair cost: £300–£800 (coolant pump replacement / wiring repair)

Replacement cost: £5,500–£10,500 (if ignored — overheating can cause engine damage)

🔴 Immediate - if the coolant warning appears or the engine overheats, stop driving immediately

Our recommendation:

The electric line for the power supply of the engine coolant pump might contact the engine fan and chafe on it, leading to electrical faults. Coolant pump and 48V hybrid system issues are a known weakness on the M256. A recall (NHTSA campaign number: 22V733000) affects certain vehicles. If you experience erratic cooling or warning lights, have the coolant pump and wiring inspected immediately.

Read the full coolant pump failure guide → /failures/m256-coolant-pump-failure/ [NEW FAILURE PAGE NEEDED]

1. Coolant Pump & 48V System Failure

Symptoms: Erratic engine cooling, warning lights, coolant loss, overheating, electric line chafing on the engine fan

Typical mileage: Any mileage — a recall affects certain CLS vehicles with the M256 engine

Repair cost: £300–£800 (coolant pump replacement / wiring repair)

Replacement cost: £5,500–£10,500 (if ignored — overheating can cause engine damage)

🔴 Immediate - if the coolant warning appears or the engine overheats, stop driving immediately

Our recommendation:

Read the full coolant pump failure guide → /failures/m256-coolant-pump-failure/ [NEW FAILURE PAGE NEEDED]

2. Timing Chain Noise on Cold Start

Symptoms: Rattling or ticking noise from the engine for a few seconds on cold start, metallic rattle that subsides as oil pressure builds, "marbles in a can" sound

Typical mileage: 40,000–80,000 miles — can occur earlier on some examples

Repair cost: £800–£2,000 (timing chain tensioner inspection and replacement) / £2,000–£4,000 (full timing chain kit)

Replacement cost: £5,500–£10,500 (if chain jumps — full engine replacement)

🔴 Immediate - if you hear rattling on cold start, book an inspection today

Our recommendation:

Timing chain noise on cold start is a reported issue on the M256 engine. A worn or stretched chain and weak tensioner cause a metallic rattling sound for a few seconds after startup. Failed guides can cause the chains to jump, skip a tooth, and potentially snap. At the first sign of cold-start rattle, have the chain tensioners and guides inspected immediately. A £800–£2,000 preventative repair is dramatically cheaper than a £5,500–£10,500 engine replacement.

Read the full timing chain failure guide → /failures/m256-timing-chain/ [NEW FAILURE PAGE NEEDED]

3. Wiring Harness & Electrical System Faults

Symptoms: Check engine light, misfires on specific cylinders, rough running, reduced power, engine speed sensor faults

Typical mileage: Any mileage — some owners report issues within the first few years

Repair cost: £500–£2,000 (wiring harness repair or replacement; ECU diagnosis)

Replacement cost: £5,500–£10,500 (if ignored — full engine replacement)

🟠 Check - book inspection urgently if warning lights appear or misfires develop

Our recommendation:

Multiple owners have reported wiring harness issues on the M256 engine. One CLS 450 owner reported a short circuit due to the copper wiring covers becoming dry and falling out. Another owner reported ignition wiring deformation causing misfires. A known issue with the m256 engine harness can cause ECU failure for ignition coils. If you experience misfires or electrical faults, have the wiring harness and ECU inspected by a specialist with genuine MB STAR diagnostic equipment.

Read the full wiring harness failure guide → /failures/m256-wiring-harness/ [NEW FAILURE PAGE NEEDED]

4. Carbon Buildup on Intake Valves

Symptoms: Rough idle, misfires, hesitation during acceleration, reduced power, poor fuel economy

Typical mileage: 50,000–80,000 miles

Repair cost: £400–£800 (walnut blasting / intake cleaning)

Replacement cost: N/A — repair only (does not progress to engine replacement if addressed)

🟡 Monitor - book inspection if symptoms develop

Our recommendation:

Like all direct-injection engines, the M256 is prone to carbon buildup on intake valves. Mercedes recommends walnut blasting every ~60,000 miles. If you experience rough idle, misfires or hesitation, have the intake valves inspected. Regular use of premium fuel and occasional spirited driving can help mitigate carbon buildup.

Read the full carbon buildup guide → /failures/carbon-buildup-intake/ [NEW FAILURE PAGE NEEDED]

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CLS 450 / 500 — 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 450 / 500 — 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 SenseCoolant pump / 48V system failure✅ Yes£300–£800May be covered by recall; catch early to prevent overheating damageTiming chain rattle (early detection)✅ Yes£800–£2,000Preventative — catches the fault before catastrophic failureTiming chain jumped❌ Usually NoEngine replacementFull engine replacement is the only reliable fixWiring harness / electrical faults✅ Yes£500–£2,000Specialist diagnosis and repair requiredCarbon buildup✅ Yes£400–£800Walnut blasting restores performance

Buying This CLS 450 / 500? Check These First

Buy If

Full service history with regular oil changes using MB-approved oil
No cold-start rattling — this is a critical warning sign
Recall work completed (coolant pump wiring harness)
Remainder of manufacturer warranty or extended warranty in place

Avoid If

Cold-start rattle with no documented timing chain work
Check engine light or misfire codes with no diagnosis
Coolant warning light or overheating history
Poor service history — the M256 is a complex engine that needs proper maintenance

Closing Verdict

The CLS 450 and 500 are powered by the M256 — Mercedes' return to the inline-six layout, delivering 362–388 hp with 48V mild-hybrid assistance. The engine is considered "robust and reliable" when serviced regularly. However, it has documented weaknesses: coolant pump and 48V system failures (subject to recall), timing chain noise on cold start, wiring harness faults, and carbon buildup. Buy a well-maintained example with full service history and no rattling. Without history? Proceed with caution — the repair bills can be substantial.

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

→ Compare CLS 450 / 500 engine prices before you buy

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Buying Checklist

Before you buy a used Mercedes CLS 450 or 500 (or before you commit to an engine replacement), check these:

Cold Start — Listen for rattling or ticking from the engine (timing chain warning — this is critical)

Coolant System — Check for recall completion (coolant pump wiring harness); check for coolant loss

Service History — Including the M256 engine and confirmed oil change intervals (MB 229.51 spec)

Engine Warning Light — Have codes been read? Look for timing, misfire or electrical codes

Wiring Harness — Any misfires or electrical faults? Check for ignition wiring deformation

Carbon Buildup — Any hesitation or rough idle? (Intake valve carbon)

48V System — Check for any hybrid system warnings

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.

M256 (CLS 450)

Years2018–2023

Power362 hp (270 kW) / 369 lb-ft (500 Nm)

2025 Enquiries— (pattern data)

Avg. Recon Cost£4,500–£7,500【MB-QUOTE】

Engine CodeYearsPower2025 EnquiriesAvg. Recon Cost

M256 (CLS 450)2018–2023362 hp (270 kW) / 369 lb-ft (500 Nm)— (pattern data)£4,500–£7,500【MB-QUOTE】

M256 (CLS 500)2018–2023381–388 hp (280–285 kW) / 384 lb-ft (520 Nm)— (pattern data)£4,500–£7,500【MB-QUOTE】

Technical Specs

M256 3.0L

Displacement2,999 cc (3.0L)

Cylinders6-cylinder inline

Bore83.0 mm / Stroke: 92.4 mm

Cylinder BlockAluminium-alloy

Cylinder HeadAluminium 24-valve DOHC

ValvetrainDOHC, 4 valves per cylinder, chain-driven

Compression Ratio10.5:1

Fuel SystemDirect injection

TurbochargerTwin-scroll turbocharger + 48V electric eBooster

Mild Hybrid48V EQ Boost (up to 21 hp / 184 lb-ft)

Power (CLS 450)362 hp @ 5,500–6,100 rpm

Torque (CLS 450)369 lb-ft (500 Nm) @ 1,600–4,000 rpm

0-60 mph4.8 seconds (4MATIC)

Top Speed130 mph (electronically limited)

Transmission9G-TRONIC 9-speed automatic

DrivetrainRear-wheel drive / 4MATIC all-wheel drive

Oil Capacity~7.5 litres (MB 229.51 spec)

Service IntervalOil ~10,000 miles

→ Compare CLS 450 / 500 engine prices for your Mercedes

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Market Intelligence

Most Common Failure

Coolant pump / 48V system failure — electric line chafing on the engine fan; subject to recall NHTSA 22V733000

Average Enquiry Mileage

30,000

–60,000 miles【PATTERN DATA】

Average Replacement Cost

£5,000

–£6,500 reconditioned, supply only【MB-QUOTE】

Most Requested Engine Code

M256【PATTERN DATA】

Peak Enquiry Season

Winter — cold starts expose timing chain issues; summer — cooling system issues become apparent【PATTERN DATA】

Regional Demand

London22%

South East18%

North West (12%)【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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As standard

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