Mercedes-Benz 420 — MOT statistics 2024

The Mercedes-Benz 420 failed 20.73% of 579 first MOT attempts in 2024 ("first-time", excluding retests). Adjusted for the age of the fleet, its initial-failure ratio is 59.6 (100 = expected for its age profile). This car fails its MOT about 40% less often than expected for its age. Small sample — treat with care.

Age-adjusted peer: Audi S5 (index 59.6) — the two published results are 0.0 index points apart.

Mercedes-Benz 420 common problems: what actually fails the MOT

Lamps, reflectors and electrical equipment are well below the reference at 17.5 per 100 tests, against 29.5 per 100 tests across UK vehicles of the same ages.

higher bar = fails more oftenrate per 100 tests

family vs national — matched to UK vehicles of the same ages — top component groups, per 100 tests. Legend: This family · National avg. Combined labels: family / national average. A gap here reflects how these vehicles are driven, used and maintained as well as how they were built — an MOT defect is a test outcome, not a breakdown.

Named defects ranked by how much more often they appear than the national average — not by raw rate.

Mercedes-Benz 420 MOT advisories: what gets flagged

Suspension is highest at 29.3 per 100 tests; Steering is lowest at 7.0 per 100 tests — the gap is 22.3 per 100 tests.

higher bar = flagged more oftenadvisory items per 100 initial tests

Advisories are wear the tester noted but didn't fail — what will need money soon.

If a car like this fails, does it fail again?

A failure rate does not say whether a failure is a one-off or the start of something. This follows the same vehicles into their next ordinary MOT a year later — retests after a repair are excluded, because they follow a failure by definition. 2019-2024.

A failed test does not clearly change next year's odds here (22.2% against 16.8%): the difference is within the margin of error. The failure most likely to come back is brakes — 25.0% of those vehicles fail again next year. Some of this is the vehicle and some is the owner: a car that was let go once tends to be let go again. One caveat: a car that fails may be sold or scrapped rather than tested again, so these figures describe the vehicles that came back.

Adjusted for how much these are actually driven

The headline index adjusts for the fleet's age. Wear, though, follows mileage: a three-year-old taxi and a three-year-old garage queen sit in the same age band. These two rows compare every car with vehicles of the same age, and then with vehicles of the same age and the same annual mileage, 2019-2024, on 2,806 tests. Both stand on the six-year corpus so they are measured on identical data — compare them with each other, not with the headline index above, which is measured on 2024 alone.

Split by how much each car is driven. This is a split by annual mileage, not by body type — the MOT record does not say whether a car is a conversion. Within each band the index compares with vehicles of the same age driven as much.

Mileage changes little here (+0.2 points): this fleet is driven about as much as the vehicles it is compared with. One limit to keep in mind: mileage is partly a result of how often a car breaks down — one that spends weeks in the workshop covers fewer miles — so the mileage-matched figure can understate a real weakness. Tests with no usable odometer reading are left out of it.

How does the Mercedes-Benz 420 perform as it gets older?

Chart axes: car age, years · failure-rate scale 0–47%.

Takeaway: solid line: this family; dashed: all cars.

How dangerous are Mercedes-Benz 420 MOT failures?

Dangerous defects are well below the reference at 6.1 per 100 tests, against 11.2 per 100 tests nationally.

higher bar = fails more oftenrate per 100 tests

Chart values: Minor 18.3 · Major 52.0 · Dangerous 6.1.

Per 100 tests on this family. National average: Minor 14.2 · Major 52.8 · Dangerous 11.2 per 100.

Does the month of a Mercedes-Benz 420 MOT matter?

Feb is highest at 28.1%; Nov is lowest at 17.5% — the gap is 10.6 percentage points.

higher = larger sharecompare shares in percentage points

Chart scale: 16%–33%.

bars: Mercedes-Benz 420 · dashed: all class-4 cars

Takeaway: the spread between best and worst month is 10.6 percentage points — noticeable seasonality. Seasonality partly reflects registration-date clustering, not weather.

Before reading the shape: most of it is not about cars. Britain changes its registration plates twice a year, and 18.9% of these cars were first registered in March with another17.1% in September — 36% in two months against the 17% an even spread would give. An MOT falls due three years after first registration, so the buying calendar becomes the testing calendar. That shapes how many cars are tested each month — not how often they fail: the busiest months here are not the worst ones, so read the failure line on its own and the volume behind it separately.

Mercedes-Benz 420 average mileage by age

Does high mileage mean more Mercedes-Benz 420 MOT failures?

All ages together, split by odometer (the bands are not split by age, so higher-mileage cars are also older and this mixes wear with age): 13.1% of the lowest-mileage cars failed versus 28.8% of the highest.

line: median · band: middle half of the fleet (p25–p75)

Where in Britain the Mercedes-Benz 420 fails its MOT most

Too few tests in any single postcode area to name one, so this is the wider picture across 3,320 tests, 2019-2024.

Raw pass/fail rates, not age-adjusted: fleets differ by region. England is split north of Sheffield, between Sheffield and Oxford, and south of it.

How many miles does a Mercedes-Benz 420 last?

100,000+ miles stands out at 49.3%.

higher = larger sharecompare shares in percentage points

Of 3,317 tests of this family with a usable odometer in 2024, 49.3% had reached 100k miles. This is a cross-section of cars still presenting for MOT, not a full birth cohort.

Mercedes-Benz 420 faults that come together

Brakes + Tyres stand out at 7.19×, against 1.00× if the faults were independent.

above 1.0× = more than expectedbelow 1.0× = less

Defects in «Brakes» and «Tyres» appear together 7.19× more often than chance (30 co-occurrences in the same test; 66 pairs compared).

Lift = how many times more often both groups appear on the same test than if independent. A pair is listed from 5 tests with both faults; the lift is shown from 20, and called above chance only when its 95% interval excludes 1 (see rough estimates).

Mercedes-Benz 420 MOT repair costs

The MOT-failure repair budget per test runs from £77.69 to £147.61, against £54.85 for the test itself.

bigger number = costs morepounds per test, MOT-failing faults only

Multiply how often each component fails on this model by what that repair usually costs, and you get the amount an owner should expect to budget per test for MOT-failing faults only — roughly £78 to £148, on top of the test fee itself. It is a budgeting figure, not a quote: the spread between a set of pads and a set of discs is most of the range.

Prices are modal quotes from a hand-kept catalogue of published UK garage guides — we do not scrape the booking sites, their terms forbid it, so this list is short and updated by hand with a source and date against each line. Each line prices one typical repair in that group, not a full remedy, and only groups our catalogue actually covers are counted. The contrast worth noticing is the second card: the legal maximum for the test has been £54.85 since 2010, while the repairs it uncovers have followed inflation the whole time.

For scale from a different direction: Warrantywise, which sees used-car warranty claims rather than test failures, puts the average claim at £2,052 across the ten highest-claim models it ranks and £539 for Honda, the lowest-claim make in its index (Warrantywise index 2026, 1.6 million claims, cars aged 3–15). Those sit an order of magnitude above the figures in this table, and the gap is the point: a warranty claim is a gearbox or an engine, which almost never fails an MOT, while the faults that do fail one are corrosion, lamps and tyres, which no warranty pays for. The two numbers answer different questions and neither replaces the other.

Mercedes-Benz 420 rust problems: where they corrode

Salt-belt exposure is well below the reference at 7th percentile, against 50th percentile as the middle of published models.

higher = larger sharecompare shares in percentage points

This Mercedes-Benz 420 has too few tests to judge for corrosion once age is held constant, and its owners live in areas that are gentler than most for the models we publish.

Corrosion is the one failure mode that is as much about geography as about the car. We keep the two apart on purpose: a model can look rusty simply because it is popular in Scotland, and a model can look clean simply because it is bought in Surrey. Blending them into a single score would hide which of the two is doing the work.

Exposure is this model's tests weighted by an area corrosion index built from frost days, salt load and distance to the coast — the composite validated at r = 0.42 against corrosion defects we actually record. It is published as a rank, not a raw score, because every mainstream model skews towards low-corrosion areas for the simple reason that most of Britain's cars are in the south. Susceptibility is standardised to the national age profile, so an old fleet does not get labelled a rusty model.

What will fail next on a Mercedes-Benz 420?

Lamps, reflectors and electrical equipment after a failure at age 20+ are highest at 17.4%; Visibility after a passed at age 20+ is lowest at 4.1% — the gap is 13.3 percentage points.

higher = more likelypercentage is the observed transition probability

The same chains, cut differently: given a car's age and how its last test went, which component most often causes trouble at the following one.

The pattern that repeats across every family: a failure at one test roughly doubles the chance of a failure on the same component at the next one, and an advisory sits about half-way between a clean pass and a failure. Age bands in years; probabilities are per car, not per defect item.

Mercedes-Benz 420 ownership: what six years of MOTs show

Clean three-test streak is highest at 49.2%; Repeated the same advisory is lowest at 22.7% — the gap is 26.5 percentage points.

higher = larger sharecompare shares in percentage points

Per mile driven, not per year. Dividing failures by the miles that actually went onto the odometers between consecutive tests gives 2.215 failures per 10,000 miles, on a median of 350 miles a year across 753 chains. This is the number to compare between models — an annual rate quietly rewards cars that barely leave the drive.

Two populations hide inside one model. 49.22% of these cars strung together three or more tests with nothing recorded at all, while 22.74% carried the same advisory three times or more — the same fault, noted, ignored, noted again. When you buy used you are buying one of those two histories, and the MOT record shows which.

Odometer anomalies: 1.577% of 761 chains show a reading lower than the previous test, with a median drop of 1,200 miles. We call this an anomaly and not fraud on purpose — instrument swaps, imports, unit mix-ups and keying errors all land in the same bucket. Obvious typos (drops under 100 miles or over 90% of the reading) are already excluded.

Mercedes-Benz 420 MOT: quick answers

Mercedes-Benz 420 MOT Risk Index is well below the reference at 59.6, against 100 for a car with the same age profile.

below 100 is betterabove 100 is worse100 = normal for this model at that age

What is the Mercedes-Benz 420 first-time MOT pass rate?

79.27% of first attempts passed in 2024 (579 tests). This is the first-time figure: retests after repair are excluded. Basis: first-time, excluding retests.

Why does this number differ from other sites?

Other sites often report a pass rate that includes retests or counts all attempts. Our 20.73% figure is the first-time initial failure rate, using 579 primary tests as the denominator (first-time, excluding retests). The age-adjusted MOT initial-failure ratio 59.6 is shown separately because raw rates compare fleet age as well as cars.

Compare the Mercedes-Benz 420 with other cars

Audi S5 is highest at 59.6; Mercedes-Benz S-Class is lowest at 59.4 — the gap is 0.2 points.

below 100 is betterabove 100 is worse100 = normal for this model at that age

Same brand

All Mercedes-Benz MOT statisticsMercedes-Benz S-Class MOT statsMercedes-Benz G MOT stats

Similar risk

Audi S5 (index 59.6)Lexus RX (index 59.5)Mercedes-Benz S-Class (index 59.4)

Same size class

how Rolls-Royce Wraith compareshow Lamborghini Aventador compareshow Renault R5 compares

By area

Mercedes-Benz 420 MOT failure rate by postcode area

By failure group

Mercedes-Benz 420 body, chassis and structure failuresMercedes-Benz 420 brakes failuresMercedes-Benz 420 lamps, reflectors and electrics failuresMercedes-Benz 420 noise, emissions and leaks failuresMercedes-Benz 420 steering failuresMercedes-Benz 420 suspension failuresMercedes-Benz 420 tyres failuresMercedes-Benz 420 visibility failures