BMW 4 Series G23Convertible2020–present
Every engine fitted to the G23 convertible — from the 2.0‑litre four‑cylinder B48 in the 420i and 430i, to the 3.0‑litre straight‑six B58 in the M440i xDrive, alongside the full range of B47 and B57 diesel options for UK buyers.
Real UK costs, honest reliability data, and the 2024 facelift engine updates that keep the range fresh.
Backed by 540+ real UK B58 enquiries and 930+ B47 enquiries in 2025 [BMW‑VERIFIED].
Engine Database
Every engine fitted to the G23 convertible — from the entry‑level 420i to the range‑topping M440i xDrive, and the full diesel range including the B57 straight‑six.
Overview
The BMW 4 Series G23 is the convertible variant of the second‑generation 4 Series, unveiled in September 2020. It replaced the folding hardtop of its F33 predecessor with a fabric soft top, saving approximately 40% weight over the previous roof mechanism. Built on BMW's CLAR platform and manufactured in Dingolfing, Germany, the G23 shares its engine line‑up with the G22 coupé.
The range includes B48 four‑cylinder petrol engines in the 420i and 430i, the B58 straight‑six in the M440i xDrive, and B47/B57 diesel options in the 420d, 430d and M440d xDrive. A 2024 facelift (LCI) brought updated engine codes and the Curved Display with OS 8.5.
Key Facts
Market Intelligence
The G23 shares its engine line‑up with the G22 — B58 enquiries totalled 540 and B47 enquiries totalled 930 in 2025 [EM‑VERIFIED], with the M440i xDrive attracting the highest proportion of performance‑oriented enquiries.
04Best & Worst Engines
Best Diesel Overall
B57D30B (M440d xDrive, 2021–present)
““The 3.0‑litre twin‑turbo straight‑six diesel produces 340 PS and 700 Nm — effortless, refined, and with 48V mild‑hybrid assistance for stop‑start smoothness. It’s the diesel you buy if you want six‑cylinder torque and genuine pace.” Who it’s for: buyers who want diesel economy with M Performance pace.”
Best Petrol Overall
B58B30B / B58B30TÜ1 (M440i xDrive, 2021–present)
““BMW’s modular straight‑six masterpiece — 382 hp, 500 Nm, silky smooth, and chain‑driven with a robust bottom end. The B58 is widely regarded as one of BMW’s most reliable modern engines. In the G23 convertible, it’s the engine that defines the open‑top experience.” Who it’s for: buyers who want six‑cylinder performance and the ultimate open‑top soundtrack. (Model-wide: B58B30 is the benchmark across the BMW range — the G23 offers the best expression of it in a convertible.)”
Highest Risk
B48B20M0 (420i, early production)
““The B48 is a robust engine overall, but early production units can suffer from coolant leaks from the expansion tank and water pump plastic fittings. These are relatively inexpensive repairs, but they’re the most common B48 issues.” Who it’s for: buyers who want entry‑level 4 Series ownership with a budget for cooling system maintenance.”
Most Expensive Failure
B57D30B (M440d xDrive)
““A 340 PS twin‑turbo straight‑six diesel with complex 48V mild‑hybrid integration. If the turbochargers or hybrid system fail, repair costs can be significant — replacement cost sits at £5,000–£8,000. The B57 is a robust engine, but it’s complex.” Who it’s for: buyers who want diesel performance with a budget for high‑end maintenance.”
Best Value
B48B20O1 (430i, 2021–present)
““255 hp and 400 Nm from a 2.0‑litre four‑cylinder — the sweet spot of the petrol range. It’s significantly more powerful than the 420i, but shares the B48’s proven reliability and relatively affordable running costs.” Who it’s for: buyers who want performance without the M440i’s premium.”
Most Overlooked
B47D20B (420d, 2021–present)
““The 420d’s 2.0‑litre four‑cylinder diesel is the most efficient engine in the range, returning up to 56 mpg with 48V mild‑hybrid assistance. The B47 is chain‑driven with an accessible front‑mounted chain — none of the N47’s rear‑chain anxiety.” Who it’s for: high‑mileage buyers who prioritise economy and reliability.”
Best Diesel Overall
B57D30B
(M440d xDrive, 2021–present)
““The 3.0‑litre twin‑turbo straight‑six diesel produces 340 PS and 700 Nm — effortless, refined, and with 48V mild‑hybrid assistance for stop‑start smoothness. It’s the diesel you buy if you want six‑cylinder torque and genuine pace.” Who it’s for: buyers who want diesel economy with M Performance pace.”
Best Petrol Overall
B58B30B
/ B58B30TÜ1 (M440i xDrive, 2021–present)
““BMW’s modular straight‑six masterpiece — 382 hp, 500 Nm, silky smooth, and chain‑driven with a robust bottom end. The B58 is widely regarded as one of BMW’s most reliable modern engines. In the G23 convertible, it’s the engine that defines the open‑top experience.” Who it’s for: buyers who want six‑cylinder performance and the ultimate open‑top soundtrack. (Model-wide: B58B30 is the benchmark across the BMW range — the G23 offers the best expression of it in a convertible.)”
Highest Risk
B48B20M0
(420i, early production)
““The B48 is a robust engine overall, but early production units can suffer from coolant leaks from the expansion tank and water pump plastic fittings. These are relatively inexpensive repairs, but they’re the most common B48 issues.” Who it’s for: buyers who want entry‑level 4 Series ownership with a budget for cooling system maintenance.”
Most Expensive Failure
B57D30B
(M440d xDrive)
““A 340 PS twin‑turbo straight‑six diesel with complex 48V mild‑hybrid integration. If the turbochargers or hybrid system fail, repair costs can be significant — replacement cost sits at £5,000–£8,000. The B57 is a robust engine, but it’s complex.” Who it’s for: buyers who want diesel performance with a budget for high‑end maintenance.”
Best Value
B48B20O1
(430i, 2021–present)
““255 hp and 400 Nm from a 2.0‑litre four‑cylinder — the sweet spot of the petrol range. It’s significantly more powerful than the 420i, but shares the B48’s proven reliability and relatively affordable running costs.” Who it’s for: buyers who want performance without the M440i’s premium.”
Most Overlooked
B47D20B (420d, 2021–present)
““The 420d’s 2.0‑litre four‑cylinder diesel is the most efficient engine in the range, returning up to 56 mpg with 48V mild‑hybrid assistance. The B47 is chain‑driven with an accessible front‑mounted chain — none of the N47’s rear‑chain anxiety.” Who it’s for: high‑mileage buyers who prioritise economy and reliability.”
Engine
Typical Mileage
Common Major Failure
Repair Cost (Specialist)
Replacement Cost (Recon)
Ownership Verdict
Solid Choice -efficient, proven B48Best Petrol Value -255 hp, proven B48Best Petrol -382 hp, the M440i’s defining assetBest Diesel Value -efficient, chain accessibleSolid Choice -282 hp, six‑cylinder refinement
Performance Diesel -340 hp, complex but rewardingEngineTypical MileageCommon Major FailureRepair Cost (Specialist)Replacement Cost (Recon)Ownership VerdictSolid Choice -efficient, proven B48Best Petrol Value -255 hp, proven B48Best Petrol -382 hp, the M440i’s defining assetBest Diesel Value -efficient, chain accessibleSolid Choice -282 hp, six‑cylinder refinementPerformance Diesel -340 hp, complex but rewarding
The G23 rule
A £4,000–£7,000 B58 recon against a £25,000–£50,000 G23 sits at 8–28% of the car’s value -well within our 70% BMW threshold. Even the most expensive engine replacement (M440d at £5,000–£8,000) is only 10–32% of a well‑kept G23’s value. The G23 is a premium convertible with strong retained values -replacement is almost always financially viable. Extended warranty cover is still recommended, especially for 48V mild‑hybrid system components.
B48
Coolant Leaks (Expansion Tank / Water Pump)
Affected models:
G23 420i, 430i (B48 four‑cylinder petrol).
Typical failure mileage:
60,000–100,000 miles.
Root cause:
The B48’s cooling system uses plastic components that can become brittle with age and heat cycling -the expansion tank and water pump are the most common failure points. Coolant loss can lead to overheating if not addressed promptly.
Tiered Cost Table
Preventative (Repair)
Dealer
Specialist
£600–£1,200
£400–£800
Catastrophic (Replacement)
Dealer
Specialist
£1,500–£3,000
£1,000–£2,000
Our Recommendation
On any B48‑powered G23 over 60,000 miles, have the cooling system inspected at every service -a £400–£800 preventative repair is far cheaper than the £1,000–£2,000 cooling system overhaul or engine replacement that follows an overheat event. The B48 is otherwise a robust engine.
48V
Mild‑Hybrid System Integration
Affected models:
G23 M440i xDrive, 420d, 430d, M440d xDrive (48V mild‑hybrid equipped models).
Typical failure mileage:
Unknown -new technology.
Root cause:
The 48V mild‑hybrid system (integrated starter‑generator, 48V battery) is relatively new across the BMW range. Early issues may include battery degradation, software integration problems, or starter‑generator failures. The system provides up to 11 hp of electric assistance.
Tiered Cost Table
Preventative (Repair)
Dealer
Specialist
£500–£1,500
£300–£1,000
Catastrophic (Replacement)
Dealer
Specialist
£2,000–£4,000
£1,500–£3,000
Our Recommendation
As a relatively new system, long‑term reliability data is limited. Ensure the 48V system is covered under warranty and have it inspected at major services. The mild‑hybrid system provides efficiency benefits and smoother stop‑start operation but adds complexity.
B47
Diesel DPF Clogging (urban use)
Affected models:
G23 420d (B47 diesel).
Typical failure mileage:
60,000–100,000 miles (urban driving).
Root cause:
Like all modern diesel engines, the B47's diesel particulate filter (DPF) can clog with soot if the car is predominantly used for short urban journeys where the DPF regeneration cycle cannot complete. The B47 is otherwise a robust and proven engine with an accessible front‑mounted timing chain.
Tiered Cost Table
Preventative (Repair)
Dealer
Specialist
£400–£800
£300–£600
Catastrophic (Replacement)
Dealer
Specialist
£1,500–£3,000
£1,000–£2,000
Our Recommendation
If you're buying a diesel G23 for urban use, ensure regular motorway runs to allow passive DPF regeneration. A £300–£600 DPF clean is far cheaper than the £1,000–£2,000 replacement that follows if ignored. The B47 is otherwise a dependable engine.
Fabric
Soft Top Mechanism Wear
Affected models:
All G23 convertibles (2020–present).
Typical failure mileage:
60,000+ miles (age‑related).
Root cause:
Unlike the F33's folding hardtop, the G23 uses a fabric soft top -this is lighter and simpler but still has a hydraulic mechanism and microswitches that can wear with age. The fabric itself can also wear, particularly at crease points.
Tiered Cost Table
Preventative (Repair)
Dealer
Specialist
£500–£1,000
£300–£700
Catastrophic (Replacement)
Dealer
Specialist
£2,000–£4,000
£1,000–£2,500
Our Recommendation
Have the roof mechanism inspected at every major service on any G23 over 60,000 miles -a £300–£700 preventative service is dramatically cheaper than the £1,000–£2,500 full overhaul that follows if ignored. The fabric roof is simpler than the F33's hardtop but still requires maintenance.
VariantEngine Code
Used (Supply)
Reconditioned (Supply)
Rebuilt (Supply)
Labour Hours
Diesel Variants
Petrol Variants
Scope note: All variants listed are G23 convertible only. The 420i uses the B48B20M0 (184 PS). The 430i uses the B48B20O1 (258 PS). The M440i xDrive uses the B58B30B (pre‑LCI, 382 PS) or B58B30TÜ1 (LCI, 382 PS). The 420d uses the B47D20B (190 PS). The 430d uses the B57D30A (282 PS). The M440d xDrive uses the B57D30B (340 PS). All diesel engines feature 48V mild‑hybrid technology. The 420i, 430i, 420d, 430d and M440d come standard with rear‑wheel drive but can be configured with xDrive; the M440i comes with xDrive only.
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£3,500–£6,500 supply only
[BMW-QUOTE]
Most Common Failures
Live FeedBMW M440i G23
B58
London
Coolant system check4
enquiries
Updated MonthlyBMW 420d G23
B47
ManchesterDPF regeneration3
enquiries
Updated MonthlyBMW 420i G23
B48
BirminghamCoolant leak2
enquiries
Updated MonthlyMost Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£3,500–£6,500 supply only
[BMW-QUOTE]
Most Common Failures
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£3,500–£6,500 supply only
[BMW-QUOTE]
Most Common Failures
Diesel Variants
Petrol Variants
Scope note: All variants listed are G23 convertible only. The 420i uses the B48B20M0 (184 PS). The 430i uses the B48B20O1 (258 PS). The M440i xDrive uses the B58B30B (pre‑LCI, 382 PS) or B58B30TÜ1 (LCI, 382 PS). The 420d uses the B47D20B (190 PS). The 430d uses the B57D30A (282 PS). The M440d xDrive uses the B57D30B (340 PS). All diesel engines feature 48V mild‑hybrid technology. The 420i, 430i, 420d, 430d and M440d come standard with rear‑wheel drive but can be configured with xDrive; the M440i comes with xDrive only.
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£3,500–£6,500 supply only
[BMW-QUOTE]
Most Common Failures
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£3,500–£6,500 supply only
[BMW-QUOTE]
Most Common Failures
Most Requested Engines (2025)
Most Requested Variants
Average Replacement Cost
£3,500–£6,500 supply only
[BMW-QUOTE]
Most Common Failures
Live FeedBMW M440i G23
B58
LondonCoolant system check4
enquiries
BMW 420d G23
B47
ManchesterDPF regeneration3
enquiries
BMW 420i G23
B48
BirminghamCoolant leak2
enquiries
Updated MonthlyTrusted G23 Engine Suppliers
every garage and engine rebuilder in our network is vetted, rated and reviewed by real 4 Series owners, as part of the Engine Finders network.
Compare G23 Engine Replacement Companies
see prices, warranty terms and supply & fit options side‑by‑side before you commit.
UK Engine Replacement Services for G23
covering every variant from 420i to M440d xDrive, with supply‑only or full supply & fit available nationwide.
Find the best BMW 4 Series G23 engine replacement UK drivers rely on. Compare real prices from trusted specialists on EnginesMarket today and get back on the road with confidence.
Compare BMW 4 Series G23 Engine Prices on EnginesMarket