How reliable is the Mercedes-Benz 420 compared with other cars?

We do not publish a comparison for the Mercedes-Benz 420, because the cars of this model that have been tested do not cover a wide enough range of ages and mileages. Its raw pass rate is 80.3% across 573 tests, but comparing a raw rate between models mostly ranks them by how new they are rather than how good they are. The failure and advisory patterns further down the page are still drawn from real tests and are worth reading.

What is the most common MOT failure on a Mercedes-Benz 420?

The item recorded most often is "A suspension joint dust cover missing or no longer prevents the ingress of dirt etc", which appears in 24.2% of failed Mercedes-Benz 420 tests. Because one failed test can list several items, these shares do not add up to 100%.

What do testers most often warn about on a Mercedes-Benz 420?

The most common advisory is "Fluid leaking excessively and likely to harm the environment or to pose a safety risk to other road users", noted on 12.4% of all Mercedes-Benz 420 tests. An advisory is not a failure: it flags something that will likely need attention before the next test.

What mileage does a Mercedes-Benz 420 usually have at its MOT?

The average odometer reading recorded at test is 99,892 miles. This is the mileage across all tested examples of every age, not an annual figure, so treat it as a description of the fleet rather than a target for one car.

Are most Mercedes-Benz 420 models petrol or diesel?

Of the Mercedes-Benz 420 cars tested, 99% are petrol, 1% are diesel. Fuel type affects which items tend to fail, particularly emissions and exhaust components.

Model reliability file

Mercedes-Benz 420 MOT pass rate & common failures

From 573 recorded MOT tests of the Mercedes-Benz 420, published by DVSA.

Data updated 29 July 2026

Quick answer

Across 573 MOT tests, 80.3% of Mercedes-Benz 420 tests ended in a pass. This figure is not adjusted for age or mileage, so it is not a reliability ranking: the Mercedes-Benz 420 fleet does not yet cover a wide enough spread of ages and distances to compare it fairly against other models. Its most commonly recorded failure is suspension joint dust cover missing or no longer prevents the ingress of dirt etc, which appears in 24.2% of failed tests. Cars of this model average 99,892 miles on the odometer when tested.

At a glance

Recorded pass rate

80.3%unadjusted, not comparable573 tests

We are not publishing a ranking for the 420. The examples tested do not cover a wide enough spread of ages and mileages to compare it fairly, and an unadjusted pass rate mostly sorts models by how recently they were bought rather than how well they were built. The failure and advisory patterns below come from the same 573 tests and are worth reading on their own terms.

DVSA recorded wording

Most common failure items

Share of failed Mercedes-Benz 420 tests in which each item was recorded. One failed test often lists several items, so these do not sum to 100%.

1.A suspension joint dust cover missing or no longer prevents the ingress of dirt etc24.2%
23 occurrencesSuspension joint dust cover missing or split explained
2.The aim of a headlamp is not within limits laid down in the requirements17.9%
17 occurrencesHeadlamp aim incorrect explained
3.Emissions levels exceed default limits13.7%
13 occurrencesEmissions above the permitted limit explained
4.A tyre seriously damaged12.6%
12 occurrencesTyre damaged or deteriorated explained
5.Significant brake effort recorded with no brake applied indicating a binding brake11.6%
11 occurrencesBrake binding or imbalanced across an axle explained
6.A suspension joint dust cover missing or no longer prevents the ingress of dirt etc8.4%
8 occurrencesSuspension joint dust cover missing or split explained
7.Wiper blade missing or obviously not clearing the windscreen8.4%
8 occurrencesWiper or washer not working properly explained
8.A headlamp or light source missing, inoperative or more than ½ not functioning in the case of LED7.4%
7 occurrencesHeadlamp not working explained
9.Parking brake efficiency below minimum requirement7.4%
7 occurrencesParking brake efficiency too low explained
10.A lamp missing, inoperative or in the case of a multiple light source more than 1/2 not functioning6.3%
6 occurrencesLamp not working explained

Warnings without a failure

Most common advisories

Share of all Mercedes-Benz 420 tests carrying each advisory. The denominator differs from the failure list above because an advisory can be issued on a car that passed.

1.Fluid leaking excessively and likely to harm the environment or to pose a safety risk to other road users12.4%
71 tests
2.A tyre seriously damaged10.3%
59 testsTyre damaged or deteriorated explained
3.A spring or spring component fractured or seriously weakened8.6%
49 testsRoad spring broken or fractured explained
4.Brake pipe damaged or excessively corroded8.2%
47 testsBrake pipe corroded explained
5.Tyre tread depth not in accordance with the requirements4.7%
27 testsTyre worn close to the legal limit explained
6.Significant brake effort recorded with no brake applied indicating a binding brake3.8%
22 testsBrake binding or imbalanced across an axle explained
7.Exhaust system leaking or insecure3.7%
21 testsExhaust leaking or insecure explained
8.Fluid leaking excessively and likely to harm the environment or to pose a safety risk to other road users3.0%
17 tests

What gets tested

The 420 fleet

Average mileage recorded at test: 99,892
Fuel mix of tested cars: Petrol99.0%
Diesel1.0%

Check a specific Mercedes-Benz 420

A model average tells you the pattern. Its own MOT record tells you the car.

Compare within the range

Other Mercedes-Benz models

Mercedes-Benz A-Class — 73.3% adjusted
Mercedes-Benz C-Class — 77.0% adjusted
Mercedes-Benz E-Class — 78.5% adjusted
Mercedes-Benz GLA — 82.3% adjusted
Mercedes-Benz Vito — 71.0% adjusted
Mercedes-Benz GLC — 85.9% adjusted
All Mercedes-Benz models

Common questions

How reliable is the Mercedes-Benz 420 compared with other cars?: We do not publish a comparison for the Mercedes-Benz 420, because the cars of this model that have been tested do not cover a wide enough range of ages and mileages. Its raw pass rate is 80.3% across 573 tests, but comparing a raw rate between models mostly ranks them by how new they are rather than how good they are. The failure and advisory patterns further down the page are still drawn from real tests and are worth reading.
What is the most common MOT failure on a Mercedes-Benz 420?: The item recorded most often is "A suspension joint dust cover missing or no longer prevents the ingress of dirt etc", which appears in 24.2% of failed Mercedes-Benz 420 tests. Because one failed test can list several items, these shares do not add up to 100%.
What do testers most often warn about on a Mercedes-Benz 420?: The most common advisory is "Fluid leaking excessively and likely to harm the environment or to pose a safety risk to other road users", noted on 12.4% of all Mercedes-Benz 420 tests. An advisory is not a failure: it flags something that will likely need attention before the next test.
What mileage does a Mercedes-Benz 420 usually have at its MOT?: The average odometer reading recorded at test is 99,892 miles. This is the mileage across all tested examples of every age, not an annual figure, so treat it as a description of the fleet rather than a target for one car.
Are most Mercedes-Benz 420 models petrol or diesel?: Of the Mercedes-Benz 420 cars tested, 99% are petrol, 1% are diesel. Fuel type affects which items tend to fail, particularly emissions and exhaust components.

How this was worked out

Method and limits

Figures come from the DVSA anonymised MOT testing dataset, published under the Open Government Licence. Only class 4 initial tests are counted: retests are excluded so a car appears once, and abandoned tests are excluded entirely. No adjusted rate is shown because this model's tested examples do not cover enough of the national age and mileage range for standardisation to mean anything.

An MOT measures whether a car met a legal standard on the day. It is not a breakdown record, a service history or a running-cost estimate, and a pass rate is a property of a fleet rather than a prediction about one car.