How much does a BMW 4 Series 420i engine replacement cost in the UK?

A reconditioned 420i engine typically costs £3,200–£7,500 supply only depending on generation, with the current B48B20 sitting at £3,500–£7,500. Fitting adds £650–£1,300. Use our calculator for a figure specific to your car.

Is the BMW 4 Series 420i engine reliable?

It depends entirely on which generation you're asking about. The N20 (F32, pre-2020) has a well-documented timing chain tensioner issue - pre-2016 cars are the highest risk. The B48 (G22, 2020+) is significantly more reliable, with the timing chain moved to the front of the engine. Across both, oil-change discipline is everything.

What are the common problems with the BMW 420i?

N20 timing chain wear (the defining weak point of the F32 generation), N20 oil filter housing gasket failure, B48 coolant leaks and PCV system issues, and valve cover gasket failure (both engines). See the Era Map above for generation-specific advice.

Is it worth replacing a BMW 420i engine, or should I scrap the car?

Depends on vehicle value. Against a well-kept £8,000–£12,000 F32 420i, a £4,000–£4,500 recon is 35–50% of the car's value - often worth it. Against a £20,000+ G22, a £5,000 recon is 25% - worth it. The BMW scrap threshold of 70% means evaluating the whole vehicle and its condition before committing.

How do I find my BMW 420i engine code?

Listed on the VIN plate and vehicle documentation. As a rule of thumb: F32/F33/F36 (2013–2020) = N20B20; G22/G23/G26 (2020–present) = B48B20. Tell us your registration date and we'll confirm it for free.

Can I repair the N20 timing chain, or does it need replacement?

Yes - if caught early. A preventative chain, tensioner and guide replacement costs £800–£1,800. However, if the chain snaps, a full engine replacement (£3,200–£6,500) is usually required. At the first sign of cold-start rattle, book an inspection immediately. Pre-2016 cars are the highest risk.

BMW 4 Series 420i2013–presentN20 / B48 2.0 Turbo

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BMW 4 Series 420i Engines - from £1,800 to £7,500Common codes: N20B20, B48B20

BMW 4 Series 420i Engines - from £1,800 to £7,500· Common codes: N20B20, B48B20

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The 4 SERIES 420i THROUGH TIME - Era Map

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

Era Note

N20 era - timing chain tensioner is the defining weak point; 2016+ revisions improved durability

GenerationYearsEngine CodeReliabilityRecon CostEra Note

4 Series F32/F33/F36 BMW 320d 4 Series F32/F33/F36

2013–2020N20B20 (2.0L 4-cyl turbo, 184 bhp)★★★½☆ [NEWLY ASSIGNED - see Data Note]£3,200–£6,500 [PLACEHOLDER - confirm vs cost sheet]N20 era - timing chain tensioner is the defining weak point; 2016+ revisions improved durability

4 Series G22/G23/G26 BMW 320d 4 Series G22/G23/G26

2020–presentB48B20 (2.0L 4-cyl turbo, 184 bhp)★★★★☆ [NEWLY ASSIGNED - see Data Note]£3,500–£7,500 [PLACEHOLDER - confirm vs cost sheet]Modern B48 - timing chain at the front, significantly improved durability; coolant and PCV are the main watchpoints

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

BMW 4 Series 420i Engine Replacement Costs

Supply Only£3,200–£6,500

Fitted (Indie)£4,200–£7,800

Warranty12–24 months

Best ForMost owners - best cost/warranty balance

Engine TypeSupply OnlyFitted (Indie)WarrantyBest For

Used Engine£1,800–£3,200£2,600–£4,5001–6 monthsBudget priority; vehicle under £5,000

Reconditioned Engine£3,200–£6,500£4,200–£7,80012–24 monthsMost owners - best cost/warranty balance

Rebuilt Engine£4,500–£7,500£5,500–£9,00012–24 monthsLong-term ownership; 3+ year horizon

Main Dealer New£6,500–£9,500£7,500–£11,00024–36 monthsInsurance jobs, warranty claims

Figures above reflect the current G22/B48 generation, the most common quote request today. Older generations: see the Era Map above for F32/N20-specific pricing.

Labour estimate: Fitting a 420i engine typically takes 8–12 hours at £75–£110/hr (independent) or £130–£180/hr (London/SE dealer). [PLACEHOLDER - confirm vs cost sheet]

→ Compare 420i engine prices from UK specialists

Common Problems With BMW 320d Engines

1. N20 Timing Chain Tensioner & Guide Failure (F32, pre-2016)

Symptoms: Rattling noise on cold start lasting 1–3 seconds, engine warning light, loss of power, metallic noises from the front of the engine, "Drivetrain Malfunction" warning.

Typical mileage: 60,000–120,000 miles

Repair cost: £800–£1,800 (preventative chain, tensioner and guide replacement)

Replacement cost: £3,200–£6,500 (if chain snaps and valves meet pistons)

🔴 Immediate - if you hear rattle, book an inspection today

Our recommendation:

The N20's timing chain tensioner is the defining weak point of this engine generation. Pre-2016 cars are the highest risk, with the chain guides being prone to failure. BMW revised the tensioner and guides in 2016, but the risk remains on all N20 engines to some degree. A cold-start rattle is the early warning sign - ignore it and you risk piston-valve contact and a full engine replacement. Regular oil changes with the correct BMW-approved oil are the best prevention. At the first sign of rattle, get it inspected immediately.

1. N20 Timing Chain Tensioner & Guide Failure (F32, pre-2016)

Symptoms: Rattling noise on cold start lasting 1–3 seconds, engine warning light, loss of power, metallic noises from the front of the engine, "Drivetrain Malfunction" warning.

Typical mileage: 60,000–120,000 miles

Repair cost: £800–£1,800 (preventative chain, tensioner and guide replacement)

Replacement cost: £3,200–£6,500 (if chain snaps and valves meet pistons)

🔴 Immediate - if you hear rattle, book an inspection today

Our recommendation:

2. N20 Oil Filter Housing Gasket Failure

Symptoms: Oil leak from the oil filter housing area, oil warning light, low oil pressure, oil on the front of the engine.

Typical mileage: 40,000–80,000 miles

Repair cost: £300–£800 (oil filter housing gasket replacement)【PLACEHOLDER】

Replacement cost: N/A - repair only

🟡 Monitor - book inspection soon; can lead to oil starvation

Our recommendation:

The N20's oil filter housing gasket is prone to failure, causing oil leaks that can damage other components if left unaddressed. The gasket becomes brittle and cracks over time. A leaking oil filter housing can also lead to oil starvation, which is catastrophic for the engine. Regular inspections can catch this before it becomes a major issue.

3. B48 Coolant Leaks & PCV System Issues (G22)

Symptoms: Low coolant warning, coolant loss with no visible external leak, sweet smell from engine bay, rough idle, white smoke from exhaust (PCV).

Typical mileage: 40,000–80,000 miles

Repair cost: £300–£1,200 (coolant system diagnosis and repair, PCV diaphragm repair)

Replacement cost: N/A - repair only

🟠 Check - book inspection urgently if coolant loss is detected

Our recommendation:

The B48 is a significant improvement over the N20, but it has its own quirks. Coolant leaks (often from the coolant reservoir, water pump, or charge cooler) are more common than in the N20. The PCV system is also a known weak point, with the valve and diaphragm being prone to failure and causing drivability issues. BMW has released several revised parts for these issues. If you notice coolant loss or rough idle, have the system inspected immediately.

4. Valve Cover Gasket Failure (Both N20 and B48)

Symptoms: Oil leak from the top of the engine, burning oil smell, visible oil seepage around the valve cover.

Typical mileage: 60,000–100,000 miles

Repair cost: £400–£900 (valve cover gasket replacement)【PLACEHOLDER】

Replacement cost: N/A - repair only

🟡 Monitor - book inspection soon; oil leaks can damage other components

Our recommendation:

Both the N20 and B48 can suffer from valve cover gasket failure. The gasket is made of rubber and has a tendency to become brittle and crack over time due to heat exposure. Obvious oil leaks are the most common symptom. Leaking oil can saturate and degrade vacuum, coolant, and air intake hoses. Regular inspections can catch this before it becomes a major issue.

5. VANOS Solenoid Wear (N20)

Symptoms: Rough idle, hesitation on acceleration, reduced power, check engine light.

Typical mileage: 70,000–100,000 miles

Repair cost: £300–£700 (VANOS solenoid replacement)【PLACEHOLDER】

Replacement cost: N/A - repair only

🟡 Monitor - book inspection soon; can affect variable valve timing

Our recommendation:

VANOS solenoid failure due to O-ring wear can affect variable valve timing on the N20. If you experience rough idle or hesitation, have the VANOS system inspected promptly.

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BMW 420i - 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.

BMW 420i - 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 SenseN20 timing chain rattle (preventative)✅ Yes£800–£1,800Catch it early - far cheaper than engine replacementOil filter housing gasket (N20)✅ Yes£300–£800Affordable preventative maintenanceB48 coolant leak✅ Yes£300–£1,200Diagnosis and repair is straightforwardValve cover gasket failure✅ Yes£400–£900Standard maintenance; replace before oil damages other componentsVANOS solenoid failure (N20)✅ Yes£300–£700Replacement is straightforwardSnapped timing chain (N20)❌ Usually NoEngine replacementPiston-valve contact means little is left to repair

Buying This 420i? Check These First

Buy If

Full service history with oil changes at 8,000–10,000 mile intervals
N20 cars (pre-2020): timing chain tensioner inspected or replaced (2016+ cars are lower risk)
B48 cars (2020+): no coolant loss and PCV system functioning correctly
No cold-start rattle on N20 cars
B48 cars: no coolant loss or sweet smell from engine bay

Avoid If

No proof of regular oil changes - these engines are unforgiving of neglect
N20 with cold-start rattle - timing chain work is imminent and expensive
Any coolant loss with no diagnosis - B48 cooling system repairs can be complex
Any engine warning light without diagnosis - the N20 and B48 can hide expensive faults
Excessive oil consumption - indicates potential piston ring or valve seal issues

Closing Verdict

The BMW 420i offers a compelling entry into the 4 Series range - sleek styling, strong performance from the 2.0-litre turbo, and excellent fuel economy. The N20-powered F32 generation is a capable car but demands timing chain vigilance, particularly on pre-2016 examples. The B48-powered G22 generation is a significant step forward in reliability and refinement, with the timing chain moved to the front of the engine. Buy on service history, maintain on a shortened schedule, and this is an excellent premium coupé.

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

→ Compare 420i 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 420i engine prices before you buy

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Buying Checklist

Before you buy a used BMW 420i (or before you commit to an engine replacement), check these:

Cold Start

Listen for rattle lasting more than 1–2 seconds (N20 timing chain warning)

Service History

Including which engine era (N20/B48) and confirmed oil-change discipline (8k–10k miles max)

Oil Level

Check dipstick; excessive consumption indicates potential issues

Oil Leaks

Check for oil around the oil filter housing (N20) or valve cover (both)

Coolant Level

Check cold and hot; any loss indicates B48 cooling system issues

Idle Quality

Any roughness? PCV or VANOS issues may be present

Engine Warning Light

Have codes been read? Timing, cooling, or VANOS codes are most common

MOT History

Check for emissions failures (often related to fuel system or PCV 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.

BMW 320d Engine Codes

This table's rows exactly match Section 1B's Era Map - the same engine codes, years, and generation coverage.

N20B20 (F32/F33/F36)

Years2013–2020

Power184 bhp (135 kW) @ 5,000 rpm · 270 Nm @ 1,250–4,500 rpm

2025 Enquiries- (pattern data)

Avg. Recon Cost£3,200–£6,500 [PLACEHOLDER]

Engine CodeYearsPower2025 EnquiriesAvg. Recon Cost

N20B20 (F32/F33/F36)2013–2020184 bhp (135 kW) @ 5,000 rpm · 270 Nm @ 1,250–4,500 rpm- (pattern data)£3,200–£6,500 [PLACEHOLDER]

B48B20 (G22/G23/G26)2020–present184 bhp (135 kW) @ 5,000 rpm · 300 Nm @ 1,350–4,300 rpm- (pattern data)£3,500–£7,500 [PLACEHOLDER]

Technical Specs

Current B48B20 generation

Displacement1,998cc

Cylinders4-cylinder inline

Compression Ratio11.0:1

Fuel SystemDirect injection (Bosch Di-Motronic)

TurbochargerSingle twin-scroll turbocharger with air-to-liquid intercooler

ValvetrainDOHC with Valvetronic and Double VANOS

→ Compare 420i engine prices for your BMW

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Market Intelligence

Most Common Failure

N20 timing chain wear

35%

of 420i enquiries cite chain or timing concerns【PATTERN DATA】

Average Enquiry Mileage

50,000

–80,000 miles【PATTERN DATA】

Average Replacement Cost

£4,800

reconditioned, supply only【PLACEHOLDER】

Most Requested Engine Code

N20B20

the F32-era 420i is the most common 420i on UK roads【PATTERN DATA】

Peak Enquiry Season

Winter

cold starts expose timing chain and cooling issues【PATTERN DATA】

Regional Demand

South East22%

London18%

North West (13%)【PATTERN DATA】

Regional Demand Breakdown

22%South East

18%London

BMW 320d - 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.

Why UK BMW 4 Series 420i Owners Trust BMWEngines.uk

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12-Month Warranty

As standard

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