2020 BMW 420: common problems and what to check
What gets flagged at MOT on 2020 BMW 420s, from 3,067 tests in 2025, plus known faults and what to look at before you buy one.
What gets flagged at MOT
The share of 2020 BMW 420 MOT tests where testers flagged each area, as a fail or an advisory. Rough prices are typical independent-garage costs.
- Tyres26% of testsRough price £70–£150
At the viewing: Check the tread on all four tyres, including the inside edges, and look for cracks or bulges.
- Brakes15% of testsRough price £150–£350
At the viewing: Look for a lip on the disc edges and feel for judder or pulling when braking firmly from about 40mph.
- Suspension4% of testsRough price £120–£400
At the viewing: Listen for knocks over speed bumps, and look behind each front wheel for a snapped coil spring.
- Wipers, windscreen and mirrors4% of testsRough price £15–£250
At the viewing: Check for chips in the driver's view, and test the wipers and washers.
- Lights and electrics3% of testsRough price £10–£80
At the viewing: Ask a friend to check every light, including brake lights and indicators, while you operate them.
- Seats and seatbelts1% of testsRough price £80–£250
At the viewing: Pull each seatbelt sharply. It should lock, and retract cleanly.
Known faults
Reported by owners, mechanics and buying guides for this model and age. Check which engine the car has, as some apply to one engine only.
- Timing chain wear2.0 petrol and diesel
The four-cylinder petrol and diesel engines (shared with the 3 Series) can suffer timing chain wear, especially with late oil changes. A failed chain is an engine-out repair.
At the viewing: Listen for a rattle or whine from the back of the engine, especially from cold. Ask for oil changes on time.
Source: Parkers: BMW 3 Series (2012-2019) used review (the 4 Series shares its engines and running gear)
- Blocked EGR valve or particulate filter (diesel)Diesel
Diesels used mostly for short trips can block the EGR valve or clog the particulate filter, causing poor running.
At the viewing: Ask what journeys it’s done and watch for warning lights or a lack of power.
If it needs doing: DPF clean or regeneration, about £160–£500. ClickMechanic
Source: Parkers: BMW 3 Series (2012-2019) used review (the 4 Series shares its engines and running gear)
- Noisy differential
A whine from the rear as the car passes you can mean the differential has been neglected.
At the viewing: Ask someone to drive it past you slowly and listen for a whine from the back.
Source: Parkers: BMW 3 Series (2012-2019) used review (the 4 Series shares its engines and running gear)
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Other years
| Year | MOT tests | Most flagged |
|---|---|---|
| 2019 | 7,511 | Tyres 28% |
| 2018 | 10,218 | Tyres 28% |
| 2017 | 11,165 | Tyres 29% |
| 2016 | 11,181 | Tyres 29% |
| 2015 | 10,034 | Tyres 31% |
| 2014 | 7,599 | Tyres 31% |
| 2013 | 537 | Tyres 33% |
Where this comes from: the DVSA's anonymised results for every MOT test in Great Britain in 2025, counting tests of 2020 BMW 420s registered in 2020. "Flagged" includes advisories, so it isn't the same as a fail rate. How we score