1991 Mercedes-Benz Sl — MOT failures & pass rate
The 1991 Mercedes-Benz Sl passed 87.6% of its 667 recorded MOT tests. That is 6.4 percentage points better than coupes & convertibles of a similar age, which fail at 18.8%. Failures cluster in lighting & signalling, visibility and suspension.
What the MOT record shows
Lighting & signalling dominates the failure mix at 10.2% of all tests — 6.4 points clear of visibility, so it is the first thing to check.
At component level the recurring items are headlamp aim, position lamp and wipers — headlamp aim alone was recorded 39 times.
Mileage matters less than usual here: the 1991 Sl fails 7.0% of the time in the 0-30k band and 9.7% at 150k+, a spread of only 2.7 points.
Advisories point at the next bill rather than this one: suspension was flagged on 19.3% of tests without failing them.
The trend is worsening — 11.4% of tests failed in 2021 against 16.1% in 2025, the usual pattern as a fleet ages.
How it compares
Top failure categories
| # | Category | % of tests affected | Tests |
|---|---|---|---|
| 1 | Lighting & signalling | 10.2% | 68 |
| 2 | Visibility | 3.8% | 25 |
| 3 | Suspension | 3.3% | 22 |
| 4 | Brakes | 3.1% | 21 |
| 5 | Emissions & environmental | 2.3% | 15 |
| 6 | Steering | 1.7% | 11 |
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 | 39 |
| 2 | Position lamp | 24 |
| 3 | Wipers | 16 |
| 4 | Headlamp | 16 |
| 5 | Stop lamp | 14 |
| 6 | Washers | 10 |
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 | 19.3% | 129 |
| 2 | Brakes | 18.6% | 124 |
| 3 | Emissions & environmental | 14.4% | 96 |
| 4 | Tyres | 13.0% | 87 |
| 5 | Steering | 8.3% | 55 |
| 6 | Body, structure & corrosion | 6.6% | 44 |
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 | 43 | 7.0% |
| 30-60k | 144 | 10.4% |
| 60-90k | 197 | 10.2% |
| 90-120k | 151 | 13.9% |
| 120-150k | 99 | 20.2% |
| 150k+ | 31 | 9.7% |
Trend over time
| Dataset year | Tests | Fail rate |
|---|---|---|
| 2021 | 149 | 11.4% |
| 2022 | 159 | 12.6% |
| 2023 | 134 | 9.0% |
| 2024 | 113 | 14.2% |
| 2025 | 112 | 16.1% |
What to check before buying a 1991 Sl
On a 1991 Sl, lighting & signalling is what the MOT record says goes wrong most — it appeared in 10.2% of tests. Check these areas before you commit.
- Lighting & signalling (10.2% of tests): Often a cheap bulb, but persistent issues can mean wiring or corrosion in the units. Typical repair: £10–£150.
- Visibility (3.8% of tests): Wipers, washers, mirrors and screen damage — usually inexpensive, but check for chips in the driver's line of sight. Typical repair: £15–£150.
- Suspension (3.3% 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 Sls pass their MOT?
87.6% of the 667 1991 Mercedes-Benz Sl MOT tests in this dataset passed — a 12.4% fail rate.
What is the most common MOT failure on a 1991 Mercedes-Benz Sl?
Lighting & signalling, recorded in 10.2% of tests, followed by visibility (3.8%).
Does the 1991 Sl get worse with mileage?
Its MOT fail rate rises from 7.0% in the 0-30k band to 9.7% in the 150k+ band.
Methodology & source. Based on 667 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.