Mercedes-Benz A-Class 2021: reliability & common MOT faults

At a glance

How 28,061 first tests went in 2025.

Passed first time: 90 in every 100
Did not pass first time: 10 in every 1002,901 failed first-attempt tests
The typical car: 4 yearsabout 32,916 miles
First tests counted: 28,061

What failed most often

Tyre condition (front)24 in every 100 failed tests
Tyre tread depth (front)13 in every 100 failed tests
Brake pads (rear)6 in every 100 failed tests

How we count this. Data: UK MOT open data, 2025. Only Class 4 cars on their first MOT of the year are counted. A car that fails and is fixed at the test station still counts as a fail in the first-attempt rate. These are raw results, not adjusted for age, mileage, or peer mix. The MOT does not test engine or gearbox internals.

Note:

These are patterns across 2021 Mercedes-Benz A-Class vehicles in 2025, not a prediction that any one car will have them. Faults are grouped by part of the car. They come from MOT defect codes, not manufacturer bulletins, recalls or a diagnosis of any one car.

Number plates and VIN

One fault: number plates (rear), 2.2 times as often as on similar cars.

Figures below are measured across the 2,901 failed first attempts in 2025 that had a mileage recorded, which allows a like-for-like comparison.

We only list faults with at least 20 failures and a rate of about 2.0 times that of similar cars, which keeps one-off results out of the list.

Number plates (rear)

Failed on 60 first tests2.2 times as often as similar carsPossible evidence

What it means

A number plate that is missing, illegible, showing an incorrect registration or so insecure that it is likely to fall off is Major. Testers do not measure the characters or their spacing and reject them only if they are obviously incorrect. Delamination and the position of fixing screws or bolts get particular attention, as they can prevent number plate recognition cameras identifying the vehicle.

Data: UK MOT open data, 2025. Compared with cars of the same age and mileage.

Advisories

Other things testers noted as worn or close to failing, more often than on similar cars.

An advisory is a note from the tester about something that has not failed yet. It is an early warning, not a fail.

Wheel bearings (rear)

Noted on 20 first tests12 times as often as similar carsElevated evidence

Anti-roll bar linkage ball joints (front)

Noted on 29 first tests4.5 times as often as similar carsLikely evidence

Brake fluid

Noted on 20 first tests3.3 times as often as similar carsElevated evidence

Data: UK MOT open data, 2025. Counts cover advisories recorded on failed first tests only, compared with cars of the same age and mileage.

MOT pass rates

How first tests ended, and how the pass rates are counted.

Passed first time: 90in every 100; Passed on the first test.
Fixed at the station: 1in every 100; Failed, then fixed at the test station the same day.
Failed: 9in every 100; Did not pass, and were not fixed at the test station.

First-attempt pass 89.7% of 28,061 first tests (2025): only cars that passed first time count. Official-style pass 91.3%: cars fixed at the test station also count as a pass. The 1 fixed and the 9 not fixed make the 10 in every 100 that did not pass first time.

By fuel type

First testsTypical mileagePassed first time, as testedPassed first time, same mileage

Petrol15,10029,366 miles9090

Diesel7,12337,027 miles8989

Hybrid5,83839,098 miles8990

Figures are in every 100 first tests. Cars with more miles usually fail more often, so the same-mileage figure gives every fuel the mileage mix of all the cars in this year.

Same model, other years

Passed first time in every 100, all tested in 2025.

Newer cars are younger, so look for a jump rather than the climb.

Year	Passed first time	First tests
2001	58 in every 100	416
2002	55 in every 100	654
2003	54 in every 100	924
2004	54 in every 100	879
2005	59 in every 100	2,304
2006	57 in every 100	3,559
2007	57 in every 100	5,100
2008	58 in every 100	5,851
2009	59 in every 100	6,260
2010	62 in every 100	5,713
2011	66 in every 100	5,097
2012	66 in every 100	4,998
2013	68 in every 100	14,910
2014	69 in every 100	22,196
2015	72 in every 100	25,391
2016	75 in every 100	36,342
2017	78 in every 100	39,003
2018	82 in every 100	39,403
2019	85 in every 100	48,623
2020	87 in every 100	33,980
2021 This car	90 in every 100	28,061
2022	91 in every 100	16,574

If you are buying one

What this year's MOT record means for the decision in front of you.

Mileage makes a modest difference on a 2021 A-Class.

Of 2021 A-Class vehicles with under 22,000 miles, 93 in every 100 passed their 2025 MOT first time. With 48,000 miles or more, 86 in every 100 did.

Under 22,0005,740 tests

93

22,000–29,0005,169 tests

92

29,000–37,000 Typical car6,027 tests

90

37,000–48,0005,565 tests

88

48,000 and over5,560 tests

86

What to check at your mileage

How often each fault turned up on a failed first test, by the car's mileage. Read it as: what is likely to need looking at on a car like the one in front of you.

	Under 22,000	29,000–37,000	48,000 and over
Tyre condition (front)	2.1	2.3	2.9
Tyre tread depth (front)	0.4	1.4	2.0
Brake pads (rear)	0.0	0.6	1.6
Windscreen wipers (front)	0.4	0.7	0.6
Headlight aim (front)	0.4	0.5	0.6

Faults per 100 first tests, by the car's mileage at the test.

When a car fails

65in every 100

of failed cars had one item to fix.

9in every 100

of failed cars had three or more items to fix.

4in every 100

of all first tests had a defect the tester marked dangerous (unsafe to drive away).

Every figure here is a first-attempt MOT rate from DVSA open data for this model line, for cars of this model year tested in 2025. Each mileage band holds a fifth of the tests. MOTs do not test engine or gearbox internals, so this is a pre-purchase checklist, not a full reliability score.

FAQs

Short answers about how to read this model-year guide.

Is the 2021 Mercedes-Benz A-Class reliable?

The 2021 Mercedes-Benz A-Class follows the pattern of the model years either side. Its main cause of failure was Tyre condition (front), about 24 in every 100 failed tests. In 2025, 89.7% of 28,061 first MOTs on the 2021 Mercedes-Benz A-Class passed first time (PRS: items fixed at the station the same day, counted as a fail). The official-style rate including PRS is 91.3%. Among 2,901 failed first-attempt MOT tests (test year 2025), Number plates (rear) appears more often than on similar cars (2.1% of those tests; 60 observed failures; about 2.2× more often than similar cars). There is no single reliability score here; this is a pre-purchase checklist built from DVSA open data, not a guarantee for any individual car.

What is most likely to fail on a 2021 Mercedes-Benz A-Class?

Ranked by how often they were actually recorded, the items that most often failed a 2021 Mercedes-Benz A-Class at the first attempt were Tyre condition (front) (24.2%), Tyre tread depth (front) (12.7%) and Brake pads (rear) (6.4%), each as a share of the year's failed first attempts. That is what goes wrong most often on the car. The common faults table shows a different measure: faults that are unusual compared with similar cars.

What common MOT faults stand out on the Mercedes-Benz A-Class 2021?

Among failed first-attempt tests we highlight patterns that appear more often than on similar cars. Top example: Number plates (rear) (2.1% of those tests; 60 observed failures; about 2.2× more often than similar cars). These are statistical signals, not a diagnosis of any individual car.

Why does Number plates (rear) stand out?

Number plates (rear) shows up more often than on similar cars (2.1% of those tests; 60 observed failures; about 2.2× more often than similar cars). That does not prove a design fault. Age, mileage, and how the car was used still matter. Treat it as a pre-purchase checkpoint, not a manufacturer service bulletin.

What are common problems with the 2021 Mercedes-Benz A-Class?

Common MOT problem areas for the 2021 Mercedes-Benz A-Class include Number plates (rear). These are the faults that stand out compared with similar cars, not a complete list of what can go wrong on any individual car.

How should I read advisories vs fails?

Advisories flag issues noted at the test and are not a fail rate. We show advisory patterns that appear more often than on similar cars among failed first-attempt tests, separate from common failure rows. Use them as early-warning checks, not as a pass/fail score.

What does this Mercedes-Benz A-Class 2021 MOT report cover?

This page highlights MOT failure patterns for the 2021 Mercedes-Benz A-Class (registration year) using UK DVSA open data for the selected test year. Patterns are compared with cars of a similar age and mileage. It is a buyer checklist from MOT defect statistics, not a full service history or manufacturer service-bulletin list.

Does this include engine and gearbox problems?

No. MOT tests do not cover engine internals, gearboxes, or many electronic modules. Patterns here come from MOT defect statistics only and should not be read as engine or gearbox reliability scores.

About this data

How this page is built and what it can (and can't) say.

These figures come from UK MOT open data for this model year. We surface common failure patterns against similar cars, not a full reliability score, and not a pass/fail verdict on any individual vehicle.

2024 2025

What we include

UK Class 4 cars only. Normal MOT tests (not retests); first test per vehicle per calendar year; results pass, PRS, or fail.

Same-day repairs

PRS means the car failed items that were fixed at the test station and then passed the same day. We count PRS as a first-attempt fail in headline rates so same-day repairs do not hide problems.

How we compare

We compare this model year with other Class 4 cars of similar age and mileage in the same test year. This model family is left out of the peer group so the car is not measured against itself.

What this does not cover

MOTs do not cover engine internals, gearboxes, or many electronics; this is not a full reliability score.
Common faults come from MOT defect stats, not manufacturer TSBs or recalls.
Age and mileage matching reduces, but does not remove, differences in how cars were used and maintained.
If the data for a car is too thin to say anything useful, we say so rather than guess.

Contains public sector information licensed under the Open Government Licence v3.0.