1991 Mercedes-Benz S-Class — MOT failures & pass rate
The 1991 Mercedes-Benz S-Class passed 86.6% of its 254 recorded MOT tests. Against executive cars of a similar age (19.1% fail rate), the 1991 S-Class comes out 5.7pp better. Failures cluster in lighting & signalling, brakes and suspension.
What the MOT record shows
No single area dominates — lighting & signalling (7.5%) and brakes (5.1%) are close, so failures are spread across several systems rather than one weak point.
At component level the recurring items are component mounting prescribed areas, service brake performance and exhaust system — component mounting prescribed areas alone was recorded 18 times.
A 150k+ example is materially riskier than a 60-90k one: 23.3% versus 0.0%.
Suspension is the most common advisory, noted on 20.1% of tests — work that passed today but is likely due soon.
The trend is improving — the fail rate fell from 15.0% in 2021 to 12.5% in 2025, as the worst-kept examples leave the fleet.
How it compares
Top failure categories
| # | Category | % of tests affected | Tests |
|---|---|---|---|
| 1 | Lighting & signalling | 7.5% | 19 |
| 2 | Brakes | 5.1% | 13 |
| 3 | Suspension | 4.7% | 12 |
| 4 | Emissions & environmental | 3.9% | 10 |
| 5 | Body, structure & corrosion | 2.8% | 7 |
| 6 | Visibility | 2.0% | 5 |
Most common specific failures
The exact components most often recorded as failures — more specific than the categories above.
| # | Failure item | Times recorded |
|---|---|---|
| 1 | Component mounting prescribed areas | 18 |
| 2 | Service brake performance | 8 |
| 3 | Exhaust system | 6 |
| 4 | Catalyst emissions | 6 |
| 5 | Position lamp | 6 |
| 6 | Ball joint dust cover | 5 |
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 | 20.1% | 51 |
| 2 | Brakes | 18.1% | 46 |
| 3 | Emissions & environmental | 16.9% | 43 |
| 4 | Body, structure & corrosion | 11.0% | 28 |
| 5 | Tyres | 10.6% | 27 |
| 6 | Steering | 6.7% | 17 |
Failure rate by mileage
Higher-mileage cars tend to fail more — often the most useful guide to real condition.
| Mileage band | Tests | Fail rate |
|---|---|---|
| 60-90k | 33 | 0.0% |
| 120-150k | 79 | 12.7% |
| 150k+ | 86 | 23.3% |
Trend over time
| Dataset year | Tests | Fail rate |
|---|---|---|
| 2021 | 60 | 15.0% |
| 2022 | 53 | 18.9% |
| 2023 | 52 | 9.6% |
| 2024 | 41 | 9.8% |
| 2025 | 48 | 12.5% |
What to check before buying a 1991 S-Class
The failure pattern for this year is specific enough to inspect against: lighting & signalling accounted for 7.5% of tests. Start there.
- Lighting & signalling (7.5% of tests): Often a cheap bulb, but persistent issues can mean wiring or corrosion in the units. Typical repair: £10–£150.
- Brakes (5.1% 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 (4.7% 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
How many 1991 Mercedes-Benz S-Classs pass their MOT?
86.6% of the 254 1991 Mercedes-Benz S-Class MOT tests in this dataset passed — a 13.4% fail rate.
What is the most common MOT failure on a 1991 Mercedes-Benz S-Class?
Lighting & signalling, recorded in 7.5% of tests, followed by brakes (5.1%).
Does the 1991 S-Class get worse with mileage?
Its MOT fail rate rises from 0.0% in the 60-90k band to 23.3% in the 150k+ band.
Methodology & source. Based on 254 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.