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MOT defects · Engine, exhaust and leaks

Lambda reading out of limits MOT failure: what it means and which cars get it

In DVSA MOT data for 2023, analysed by RegTail, testers recorded this as a failure on at least 0.2 in 100 initial MOT tests. The Volkswagen Fox got it most often: recorded 5.0 times per 100 of its 28,542 tests.

DVSA wording: Lambda coefficient outside the default limits or the range specified by the manufacturer.

Failures
0.2 in 100
At least 52,158 tests in 2023
Most often
Volkswagen Fox
5.0 per 100 tests

What it means

On a petrol car with a catalytic converter, the emissions test measures the air and fuel balance (lambda). A reading outside the limits usually points to a faulty oxygen sensor, an air leak, a misfire or a worn catalytic converter.

How serious it is

A major defect, so a fail.

Source: MOT inspection manual for cars and light vans, section 8.2.1.2.

  1. Major Fail
    It may affect the vehicle's safety, put other road users at risk or have an impact on the environment. Repair it immediately.

What fixing it usually involves

A diagnosis first; often an oxygen sensor or a vacuum leak, sometimes a new catalytic converter.

A general guide, not a quote: ask a garage to inspect the car.

Which models get it most

Per 100 initial MOT tests on each model, among the 26 models with at least 20,000 tests where it is among the 40 most common items. DVSA MOT data for 2023, analysed by RegTail.

ModelTestsFail
Volkswagen Fox28,542
5.0
Fiat Punto72,390
3.9
Peugeot 20658,647
3.2
Porsche Boxster33,380
2.9
Mitsubishi Colt25,654
2.3
Alfa Romeo Mito20,894
2.3
Peugeot 107119,430
2.3
Hyundai Getz23,077
2.2
Volkswagen Beetle48,614
2.1
Renault Twingo26,208
2.1

What to check at a viewing

  1. On the car
    Ask whether it has failed on emissions before, and watch for an engine warning light or rough idle.
  2. In the MOT history
    If this appears in past results, ask what was done and when, and for the receipt.

Check a specific car

Fleet averages only go so far. Enter the registration to see that car's own MOT history, mileage record and current condition, free.

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Questions

What does "Lambda reading out of limits" mean on an MOT?
On a petrol car with a catalytic converter, the emissions test measures the air and fuel balance (lambda). A reading outside the limits usually points to a faulty oxygen sensor, an air leak, a misfire or a worn catalytic converter.
Lambda reading out of limits: is it an MOT fail?
A major defect, so a fail.
Which cars are most likely to get "lambda reading out of limits" on an MOT?
In DVSA MOT data for 2023, analysed by RegTail, the models that got it most often were the Volkswagen Fox (5.0 per 100 tests), the Fiat Punto (3.9 per 100 tests), the Peugeot 206 (3.2 per 100 tests).

More engine, exhaust and leaks defects

See every MOT defect we explain or MOT pass rates by model.

Oil leakGearbox oil leakExhaust leakEngine warning light onLambda reading out of limitsAirbag warning light on

How these figures are made. We counted the defects recorded at every initial MOT test (not retests) of cars and light vans in Great Britain in 2023 in the DVSA anonymised MOT dataset, 33,842,940 tests in all. Each item counts once per test. A failure is a major or dangerous defect, including one fixed during the test. A model's rate is the share of its tests where the item was recorded.

For each model we keep its 40 most frequent items, so a model where a defect is rarer than that is not listed, and figures for all cars are a floor. Defect names and explanations are ours, checked against the MOT inspection manual; DVSA's own wording is shown under each name.

Contains public sector information licensed under the Open Government Licence v3.0. RegTail's analysis of the DVSA anonymised MOT dataset, 2023 tests.