MOT performance
BMW Z4 vs Porsche Boxster: MOT pass rates compared
In 2023–2024, 76.9% of 70,690 BMW Z4 MOT tests and 82.2% of 65,604 Porsche Boxster tests passed first time — 5.3 points in favour of the Porsche Boxster. Once age, mileage and fuel are allowed for, the Porsche Boxster fails less often than the BMW Z4: 0.55× against 0.68× the national rate for comparable cars.
70,690 BMW Z4 and 65,604 Porsche Boxster MOT tests, 2023–2024
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| Figure | Value | Basis |
|---|---|---|
| BMW Z4: first-time pass rate | 76.9% | ±0.3 pts (95%), n = 70,690 |
| Porsche Boxster: first-time pass rate | 82.2% | ±0.3 pts (95%), n = 65,604 |
| BMW Z4: failures vs similar cars | -32% | same age, mileage and fuel = 1.00 |
| Porsche Boxster: failures vs similar cars | -45% | same age, mileage and fuel = 1.00 |
| BMW Z4: dangerous defect | 7.2% | n = 70,690 |
| Porsche Boxster: dangerous defect | 3.7% | n = 65,604 |
| Average age at test | 13 vs 15.7 years | BMW Z4 vs Porsche Boxster |
Why the raw pass rates can mislead
The Porsche Boxster was tested at an older average age (15.7 years against 13), and older cars fail more often whatever they are. That is why the table above also shows failures against cars of the same age, mileage and fuel: it compares the models, not their fleets.
Failure areas side by side
Share of MOT tests that failed on each area, and in brackets how that compares with cars of the same age, mileage and fuel (1.00 = national average). The clearest difference is road wheels, where the BMW Z4 fails 10.7 times as often on an adjusted basis.
| Area | BMW Z4 | Porsche Boxster |
|---|---|---|
| Brakes | 5.5% (-34%) | 4.1% (-52%) |
| Steering | 0.3% (-85%) | 0.7% (-66%) |
| Suspension | 6.5% (-44%) | 4.2% (-61%) |
| Tyres | 5.9% (-3%) | 3.1% (-42%) |
| Road wheels | 1.5% (+320%) | 0.1% (-61%) |
| Lights, reflectors and electrical | 6.3% (-58%) | 6.5% (-54%) |
| Body, chassis and structure | 0.9% (-81%) | 1.8% (-66%) |
| Driver's view of the road | 4.0% (-31%) | 2.3% (-61%) |
| Noise, emissions and leaks | 3.1% (-36%) | 4.4% (-12%) |
| Seat belts and airbags | 1.5% (+10%) | 0.4% (-75%) |
| Registration plates and VIN | 0.4% (-7%) | 0.4% (-19%) |
| Speedometer and driving controls | 0.0% (-73%) | 0.0% (-73%) |
Pass rate at the same age
Comparing cars of the same age removes most of the fleet-age effect from the raw rates.
| Age at test | BMW Z4 | Porsche Boxster |
|---|---|---|
| 3-4 years | 90.2% (n = 5,681) | 94.4% (n = 2,772) |
| 5-6 years | 85.2% (n = 1,645) | 94.0% (n = 3,960) |
| 7-8 years | 85.0% (n = 5,360) | 93.2% (n = 4,572) |
| 9-10 years | 81.8% (n = 7,083) | 91.5% (n = 4,968) |
| 11-12 years | 81.4% (n = 7,641) | 90.3% (n = 3,660) |
| 13-15 years | 76.6% (n = 13,253) | 87.8% (n = 5,505) |
| 16+ years | 68.8% (n = 28,075) | 76.0% (n = 39,595) |
Most common failure item
- BMW Z4 · condition: has ply or cords exposed (2.09% of tests)
- Porsche Boxster · catalyst emissions: lambda reading after 2nd fast idle outside specified limits (2.93% of tests)
MOT results reflect how cars are maintained as well as how they are built, so a gap between two models is a difference in test performance, not proof that one is more reliable. For one car, its own MOT history says more than any average.
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About this data. These are MOT test results, not a measure of mechanical reliability: a car's MOT outcome reflects its condition on the day, which depends on how it has been maintained and driven as well as how it was built. Figures cover class 4 vehicles (cars and light vans), normal tests only (retests excluded), outcomes pass, fail and pass after rectification at the station; vehicles at least 2.5 years old at test. Tests from 2023-01-01 to 2024-12-30. “Compared with similar cars” means compared with the national rate for cars of the same age, mileage band and fuel. Contains public sector information licensed under the Open Government Licence v3.0. Makes and models are matched to 73 Auto's vehicle catalogue; tests that can't be matched are left out rather than guessed. See how the MOT figures are produced.