Mercedes-Benz 420 MOT pass rate & common failures
3,356 MOT tests on the Mercedes-Benz 420 (1980–2020) give it an 84.3% pass rate. Against all cars of a similar age (21.0% fail rate), the 420 comes out 5.3pp better. Its most common failure areas are lighting & signalling, brakes and suspension.
What the MOT record shows
One area stands out: lighting & signalling, recorded in 9.0% of tests against 5.7% for the next-worst category.
At component level the recurring items are headlamp aim, ball joint dust cover and service brake performance — headlamp aim alone was recorded 171 times.
A 150k+ example is materially riskier than a 0-30k one: 21.8% versus 7.1%.
Suspension is the most common advisory, noted on 19.9% of tests — work that passed today but is likely due soon.
The trend is improving — the fail rate fell from 17.1% in 2021 to 15.1% in 2025, as the worst-kept examples leave the fleet.
How it compares
By model year
Every 420 model year with a usable sample. Years with their own detailed report are linked.
| Year | Tests | Pass rate | Fail rate | Most common failure |
|---|---|---|---|---|
| 1986 | 501 | 82.0% | 18.0% | Suspension |
| 1987 | 868 | 84.5% | 15.6% | Lighting & signalling |
| 1988 | 870 | 84.4% | 15.6% | Lighting & signalling |
| 1989 | 673 | 85.3% | 14.7% | Lighting & signalling |
| 1990 | 192 | 84.4% | 15.6% | Lighting & signalling |
| 1991 | 101 | 83.2% | 16.8% | Brakes |
| 1992 | 33 | 100.0% | 0.0% | Lighting & signalling |
Most common failures (all years)
| # | Category | % of tests affected | Tests |
|---|---|---|---|
| 1 | Lighting & signalling | 9.0% | 302 |
| 2 | Brakes | 5.7% | 192 |
| 3 | Suspension | 5.5% | 183 |
| 4 | Visibility | 4.4% | 149 |
| 5 | Body, structure & corrosion | 3.7% | 124 |
| 6 | Emissions & environmental | 3.3% | 110 |
| 7 | Tyres | 1.8% | 59 |
| 8 | Steering | 1.1% | 38 |
Most common specific failures
The exact components most often recorded as failures — more specific than the categories above.
| # | Failure item | Times recorded |
|---|---|---|
| 1 | Headlamp aim | 171 |
| 2 | Ball joint dust cover | 140 |
| 3 | Service brake performance | 117 |
| 4 | Wipers | 96 |
| 5 | Component mounting prescribed areas | 93 |
| 6 | Washers | 91 |
| 7 | Non catalyst emissions | 87 |
| 8 | Position lamp | 62 |
Top advisory categories
Advisories aren't failures, but they flag work likely needed soon — useful when budgeting.
| # | Category | % of tests with advisory | Tests |
|---|---|---|---|
| 1 | Suspension | 19.9% | 666 |
| 2 | Brakes | 15.2% | 510 |
| 3 | Emissions & environmental | 14.3% | 481 |
| 4 | Tyres | 10.0% | 336 |
| 5 | Body, structure & corrosion | 9.2% | 308 |
| 6 | Steering | 7.8% | 263 |
Failure rate by mileage
Higher-mileage cars tend to fail more — often the most useful guide to real condition.
| Mileage band | Tests | Fail rate |
|---|---|---|
| 0-30k | 85 | 7.1% |
| 30-60k | 418 | 8.8% |
| 60-90k | 879 | 14.3% |
| 90-120k | 900 | 16.1% |
| 120-150k | 599 | 18.0% |
| 150k+ | 473 | 21.8% |
By fuel type
| Fuel | Tests | Fail rate |
|---|---|---|
| Petrol | 3,320 | 15.7% |
| Diesel | 36 | 13.9% |
Trend over time
| Dataset year | Tests | Fail rate |
|---|---|---|
| 2021 | 735 | 17.1% |
| 2022 | 701 | 16.3% |
| 2023 | 644 | 13.8% |
| 2024 | 642 | 15.7% |
| 2025 | 634 | 15.1% |
What to check before buying a Mercedes-Benz 420
- Lighting & signalling (9.0% of tests): Often a cheap bulb, but persistent issues can mean wiring or corrosion in the units. Typical repair: £10–£150.
- Brakes (5.7% of tests): Pads/discs are routine wear; binding, imbalance or corroded pipes are more serious — test for pulling under braking. Typical repair: £100–£350 per axle.
- Suspension (5.5% of tests): Worn drop links, bushes, springs or shocks — listen for knocks over bumps and check for uneven tyre wear. Typical repair: £150–£450 per corner.
Repair costs are rough UK ballpark ranges to set expectations, not quotes — actual prices vary widely by car, parts and garage.
Frequently asked questions
What is the MOT pass rate for the Mercedes-Benz 420?
84.3% of the 3,356 Mercedes-Benz 420 MOT tests in this dataset passed, a 15.7% fail rate, across model years 1980–2020.
What does the Mercedes-Benz 420 most often fail its MOT on?
Lighting & signalling, recorded in 9.0% of tests, then brakes (5.7%).
How much does mileage affect 420 MOT results?
Fail rates run at 7.1% for cars in the 0-30k band and 21.8% for 150k+, a 14.7-point difference.
Methodology & source. Based on 3,356 MOT tests. Dataset: DVSA MOT testing data (2021,2022,2023,2024,2025). Data last updated 2026-07-27. Figures reflect MOT-testable defects only — read the methodology for how these are calculated and what they don't measure.