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MOT defects

Nuisance: noise, emissions and leaks: 56 MOT defect wordings

DVSA MOT tests 2022–2025 · 130,233,413 MOT tests · Updated

56 nuisance: noise, emissions and leaks defect wordings were recorded 12,079,405 times in 130,233,413 MOT tests from 2022 to 2025 [1].

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All nuisance: noise, emissions and leaks wordings

Every wording in this section, most recorded first. Per 100 MOT tests at all ages.
WordingRecorded asPer 100 MOT tests
Oil leak, but not excessiveadvisory4.59
Malfunction indicator lamp inoperative or indicates a malfunctionfail1.46
Transmission oil leak, but not excessiveadvisory0.59
Petrol exhaust Lambda reading after 2nd fast idle outside specified limitsfail0.52
Emissions not tested unable to be completedfail0.29
Diesel exhaust emissions exceed manufacturer's specified limitfail0.26
Petrol exhaust carbon monoxide content after 2nd fast idle exceeds manufacturer's specified limitsfail0.20
Oil leaking excessively from enginefail0.12
Petrol exhaust emissions likely to be affected by an exhaust leakfail0.12
Petrol exhaust carbon monoxide content after 2nd fast idle exceeds default limitsfail0.12
Emissions not testedfail0.11
Petrol exhaust carbon monoxide content at idle exceeds manufacturer's specified limitsfail0.088
Fluid leak, but not excessiveadvisory0.077
Petrol exhaust hydrocarbon content after 2nd fast idle exceeds manufacturer's specified limitsfail0.075
Petrol exhaust carbon monoxide content at idle exceeds default limitsfail0.064
Diesel exhaust emits excessive smoke or vapour but not likely to obscure the vision of other road usersadvisory0.053
Diesel exhaust emissions likely to be affected by an exhaust leakfail0.052
Petrol exhaust hydrocarbon content after 2nd fast idle exceeds default limitsfail0.051
Undertray not fully secureadvisory0.047
Diesel exhaust emissions test not completed because smoke levels are significantly in excess of the specified limit valuesfail0.042
Diesel exhaust emissions exceed default limit of 1.5m-1fail0.038
Petrol exhaust emits blue smoke during accelerationadvisory0.033
Diesel exhaust on a vehicle fitted with a diesel particulate filter emits visible smoke of any colourfail0.024
Diesel exhaust emissions exceed level of metered smoke for a turbo charged enginefail0.024
Hydraulic fluid leak, but not excessiveadvisory0.023
Diesel exhaust emissions exceed default limit of 0.7m-1fail0.020
Diesel exhaust emits excessive smoke or vapour likely to obscure the vision of other road usersfail0.017
Petrol exhaust emits excessive dense blue smoke during accelerationfail0.016
Fluid leak harmful to the environment or a risk to other road usersfail0.013
Petrol exhaust emits black smoke during accelerationadvisory0.012
Sound deadening material not fully secureadvisory0.011
Undertray insecurefail0.011
Catalytic converter missing where one was fitted as standardfail0.010
Transmission oil leaking excessively from transmissionfail0.009
Petrol exhaust emissions - engine idle speed too highfail0.008
Oil leaking continuously from engine posing a serious risk to road safetyfail0.008
Petrol exhaust emissions carbon monoxide content excessivefail0.007
Diesel exhaust emissions exceed level of metered smoke for a non-turbo charged enginefail0.007
Diesel particulate filter shows evidence of tamperingfail0.007
Hydraulic fluid leaking excessivelyfail0.006
Petrol exhaust emits clearly visible black smoke during accelerationfail0.005
Fluid leaking continuously posing a serious risk to road safetyfail0.004
Emission control equipment obviously defectivefail0.004
Petrol exhaust emissions likely to be affected by an induction leakfail0.004
Diesel exhaust emissions likely to be affected by an induction leakfail0.003
Sound deadening material insecurefail0.003
Petrol exhaust emits dense blue smoke at idlefail0.003
Petrol exhaust emissions hydrocarbon content excessivefail0.003
Oxygen sensor obviously defectivefail0.002
Particulate filter obviously modifiedfail0.002
Particulate filter obviously defectivefail0.002
Catalytic converter obviously modifiedfail0.001
NOx sensor obviously defectivefail0.001
Hydraulic fluid leaking continuously posing a serious risk to road safetyfailunder 0.001
Petrol exhaust emits visible black smoke at idlefailunder 0.001
Particulate filter missing where one was fitted as standardfailunder 0.001

Was it on the record of the car you are looking at?

These figures are for every car tested. One car's MOT history, by registration, lists each advisory and fail item its testers recorded, and when.

  • Check any car's MOT history Every MOT test, fail item, advisory and mileage reading DVSA holds for that registration.
  • MOT advisory checker Paste an advisory or defect to read DVSA's wording, its grade and what tends to happen at the next MOT.

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Questions people ask

Which nuisance: noise, emissions and leaks defect do testers record most?

4.59 in 100 MOT tests record “Oil leak, but not excessive”, the most recorded wording in this section (130,233,413 MOT tests).

How many nuisance: noise, emissions and leaks wordings are fails?

47 of the 56 wordings here are fails (major or dangerous defects); the others are advisories and minor defects, which pass.

Which nuisance: noise, emissions and leaks advisory most often turns into a fail?

12.0% of catalyst emissions advisories were followed by a fail on it at the next MOT, against 1.9% for cars the same ages without one.

Buying Report £14.99 incl. VAT · one car · no subscription · Not a mechanical inspection.

Sources

  1. Driver and Vehicle Standards Agency, Anonymised MOT tests and results, 2022 to 2025. DVSA MOT data, Open Government Licence v3.0, 130,233,413 MOT tests. Accessed 7 Oct 2026.

Rates are per 100 MOT tests of class 4 vehicles (cars and light vans) at each age. Model comparisons are against all class 4 vehicles of the same ages and fuel, pooled over 2022 to 2025 tests: they are not matched on test year or on mileage. Advisory figures count pairs of consecutive MOT tests of the same vehicle: "next MOT" means its next test 300 to 500 days (about 10 to 16 months) later, and the no-advisory comparison is standardised to the advised vehicles' age bands. Repairs between MOTs and breakdowns are not in this data. How we compare