BMW 420d common problems and MOT failures

What usually goes wrong with a BMW 420d?

Known faults reported by owners, mechanics and UK buying guides, in our own words. Most apply to certain years or engines only, so check which the car has.

• Timing chain wear

  Cars registered 2013–2020 · 2.0 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)

  Cars registered 2013–2020 · 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

  Cars registered 2013–2020

  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)

Common BMW 420d MOT failures to check

Parts flagged at MOT well above the usual rate for cars the same age, in more than one year. From the DVSA’s results for every MOT done in Great Britain during 2023–2025.

• Steering rack inner joints

  up to 11.8× usual

  Stands out on cars registered in 2013, 2014, 2015, 2016 and 2017.

  Rough price £100–£450 our estimate

  At the viewing: On a straight road the steering wheel should sit straight. Listen for knocks when turning, and check the steering feels even.

• Transmission oil leaks

  up to 7× usual

  Stands out on cars registered in 2015 and 2016.

  Rough price £80–£600 our estimate

  At the viewing: Look under the engine and where the car is usually parked for fresh oil. Ask whether the leak has been looked at.

• Shock absorbers

  up to 1.9× usual

  Stands out on cars registered in 2013 and 2014.

  Rough price £200–£300 (pair, average rear to front) ClickMechanic

  At the viewing: Push down hard on each corner. It should bounce once and settle. Look for oil on the shock absorber body.

BMW 420d MOT pass rate by year

Of the years since 2005 with plenty of MOT tests, the 2013 BMW 420d did best (75% passed first time, 8 points above all 2013 cars) and the 2021 did worst (86%, 1 point below all 2021 cars). Older cars fail more, so compare each year with all cars registered the same year.

Questions to ask the seller before buying a BMW 420d

Ask before you travel to see it. A seller who knows the car will have answers, and receipts.

1. Has the timing chain been checked or replaced, and have oil changes been on time?
2. Any recent work on the steering rack inner joints? (Flagged at MOT 11.8× as often on the BMW 420d as on other cars the same age.)
3. Any recent work on the transmission oil leaks? (Flagged at MOT 7× as often on the BMW 420d as on other cars the same age.)
4. Any recent work on the shock absorbers? (Flagged at MOT 1.9× as often on the BMW 420d as on other cars the same age.)
5. Can I see the service history and receipts for any big jobs?