Performance by cohort

3 year bands · 14,385 tests

Pass rate climbs 8.6 points across the cohorts — newer B examples clear the test more reliably than the early cars.

Pre-2018 cohort 186

Pass

84.4%

Fail

14.0%

PRS

1.6%

Avg mileage at test

64,820 mi

2018–2020 cohort 11,643

Pass

91.3%

Fail

6.3%

PRS

2.1%

Avg mileage at test

28,280 mi

2021+ cohort 2,556

Pass

93.0%

Fail

4.4%

PRS

2.2%

Avg mileage at test

21,328 mi

Cohort = vehicle's first-registration year band. Same model, different generations of build.

The picture

B: a strong MOT record by UK norms

Across 7,108 MOT tests, the B returns 90.5% first-time pass — comfortably ahead of the UK fleet average. The single most-logged Major fail is a seriously damaged tyre. A defective wiper blade and windscreen damage round out the top three. Average tested mileage sits at 22,070, which is the lens to read those failure rankings through. If you own one and the next test is close, the ranked list below is a sensible pre-test checklist.

ABI Insurance Group

Group 20–32

Above average — worth comparing quotes before buying. Lower groups cost less to insure; UK fleet average is around Group 22.

Source: ABI Group Rating Panel · administered by Thatcham Research · groups cover standard variants; performance trims may sit higher. Browse all insurance groups →

20–32

out of 50

Compare quotes →

Top ten reasons for rejection.

Filter failures:

Hide wear items to see the model's real defects.

01

A tyre seriously damaged

252 occurrences · 1.8% of tests
02

Wiper blade defective

185 occurrences · 1.3% of tests
03

Windscreen or window damaged or seriously discoloured but not adversely affecting driver's view

139 occurrences · 1.0% of tests
04

Tyre tread depth not in accordance with the requirements

100 occurrences · 0.7% of tests
05

A tyre cords visible or damaged

84 occurrences · 0.6% of tests
06

a brake lining or pad worn below 1.5mm

67 occurrences · 0.5% of tests
07

Wiper blade missing or obviously not clearing the windscreen

53 occurrences · 0.4% of tests
08

An SRS malfunction indicator lamp (MIL) indicates a system malfunction

52 occurrences · 0.4% of tests
09

A tyre seriously damaged

43 occurrences · 0.3% of tests
10

A transmission shaft constant velocity joint boot severely deteriorated

24 occurrences · 0.2% of tests

Counts cover Major and Dangerous defects logged at test. Advisory items excluded so this shows why a car was rejected, not just what the tester flagged in passing.

Worst-case fix budget · top 3 failures

£140–£235

If every one of this B's most-logged Major fails hit at the same MOT, that's the real-world UK garage range. Reality is usually one or two items, not all of them. Open the estimator →

Try the calculator

Build your own retest budget.

Year-band analysis

Best year to buy. Worst to avoid.

First-time MOT pass rate split by registration band. A 8.6-point gap between bands means the year you buy Mercedes Benz B has a real effect on what turns up at the garage.

Best band to buy

93.0%

2021+ registration

the 2021-on band climbs to 93.0% — a 8.6-point improvement. Tests in this band average 21,328 miles — roughly 43K miles fewer on the clock than the older band. Failures here are mostly wear items: has a cut in excess of the…, damaged but not adversely affecting driver's view — the structural issues that drag down older examples don't appear in the top-10 for this band. Post-2020 examples are early in their MOT life and generally show the cleanest records.

Band to be cautious about

84.4%

Pre-2018 registration

On the older band (pre-2018), the data shows a 84.4% pass rate against a fleet average of 93.0% on the newer band. The main culprits logged at test: constant velocity boot severely deteriorated, damaged but not adversely affecting driver's view, and inoperative in the case of a single…. Average mileage on test for this band is 64,820 miles — high-mileage wear items are a recurring theme.

Best band to buy: 2021+ (93.0% first-time pass). Worst band to avoid: pre-2018 (84.4% pass). That's a 8.6-point spread across 186 older tests and 2,556 newer ones — year of build makes a material difference on this model.

Year-spread leaderboard →

Tools that pre-empt a retest.

Picked against this car's top failure patterns. Affiliate links to Amazon UK — we earn a small cut at no cost to you. Disclosed up-front, doesn't shape the data.

Item 01 · Amazon UK

Bosch Aerotwin wiper blades

Cheap wipers fail the test on smear streaks. Bosch Aerotwins last a winter and clear a screen properly.

Search Amazon UK

Item 02 · Amazon UK

Digital tyre-tread depth gauge

Five quid for a gauge beats £150 for a retest. UK MOT minimum is 1.6mm — most testers fail anything below 2mm to be safe.

Search Amazon UK

My Motor World · affiliate

Parts & supplies for this fix

Affiliate links — small commission, no extra cost to you.

Denso

Denso W14E Spark Plug (Single)

£1.09

View at My Motor World →

Denso

Denso W16EX-U Spark Plug (Single)

£1.09

View at My Motor World →

Denso

Denso W16FSR Spark Plug (Single)

£1.09

View at My Motor World →

Denso

Denso W9LM-US Spark Plug (Single)

£1.09

View at My Motor World →

Click Mechanic · affiliate

Book a mobile mechanic

Affiliate links — small commission, no extra cost to you.

Mobile mechanic · UK-wide

Book a mechanic at your door.

Fixed-price quotes upfront. No garage needed. Click Mechanic sends a vetted local mechanic to you — home, work, or roadside.

Get a quote →

Buying or keeping a B?

Use the failure ranking as a pre-test checklist or a haggling lever. Treat the headline pass rate as a fleet-wide trend, not a guarantee on any individual car.

If you own a B and your last MOT looked nothing like the ranked failures above, that's normal — individual cars vary widely. The ranking shows the patterns testers flag most often across the country.