MOT performance
MOT failure “Catalyst emissions: carbon monoxide content at idle exceeds manufacturer's specified limits”: how common it is and which cars get it
“Catalyst emissions: carbon monoxide content at idle exceeds manufacturer's specified limits” (major defect, noise, emissions and leaks) was recorded as a failure on 0.10% of UK MOT tests in 2023–2024: 68,745 tests, the number 91 most common failure item. It is 7.5 times as common at 10 years old (0.06%) as at 3 (0.01%). It is most common at 20 years old (0.44% of tests).
68,745 tests with this failure, 2023–2024
Check a specific car's MOT history and running costsAt a glance
| Figure | Value | Basis |
|---|---|---|
| Share of MOT tests | 0.10% | 68,745 tests, 2023–2024 |
| Rank among failure items | #91 | by number of tests |
| Most common at | 20 years old | 0.44% of tests at that age |
| Models compared | 209 | models with enough tests |
How it changes with age
By fuel
Petrol: 0.20% of tests; Hybrid: 0.00% of tests; Diesel: 0.00% of tests; Electric: 0.00% of tests. Fuel groups differ in age and mileage, so these raw shares are not like-for-like; the model tables below are.
Models that fail on it most often
Relative to cars of the same age and fuel (1.00 = national rate for this item). Only models with enough tests for the difference to be clear are ranked.
| Model | Tests | Failed on it | vs similar cars |
|---|---|---|---|
| Peugeot 308 | 274,758 | 0.20% | +313% |
| Peugeot 208 | 402,103 | 0.22% | +253% |
| Nissan 350 Z | 10,960 | 1.46% | +234% |
| Nissan X-Trail | 186,399 | 0.09% | +228% |
| Hyundai Tucson | 263,314 | 0.07% | +208% |
| Kia Sportage | 531,226 | 0.07% | +207% |
| Audi Q3 | 250,413 | 0.07% | +206% |
| SEAT Mii | 36,559 | 0.28% | +205% |
| Skoda Citigo | 104,908 | 0.25% | +186% |
| Mazda Bongo | 21,062 | 0.49% | +173% |
Models that fail on it least often
| Model | Tests | Failed on it | vs similar cars |
|---|---|---|---|
| Jaguar XK | 16,340 | 0.01% | -98% |
| Suzuki Splash | 35,745 | 0.01% | -93% |
| Suzuki Celerio | 55,281 | 0.01% | -92% |
| Toyota Verso | 56,704 | 0.01% | -89% |
| Vauxhall Agila | 73,056 | 0.04% | -87% |
| Mercedes-Benz B-Class | 145,990 | 0.01% | -86% |
| Bentley Continental | 29,070 | 0.04% | -85% |
| Renault Captur | 300,538 | 0.01% | -83% |
| Aston Martin Vantage | 15,831 | 0.04% | -82% |
| Toyota Auris | 311,459 | 0.02% | -81% |
About this defect
The wording is DVSA's, from the MOT inspection manual. A high ratio for a model reflects its MOT record, which depends on maintenance and use as well as design. For one car, its own MOT history shows whether this has come up before.
Other common noise, emissions and leaks failures
- Malfunction indicator lamp: inoperative or indicates a malfunction · 0.95% of tests
- Catalyst emissions: Lambda reading after 2nd fast idle outside specified limits · 0.53% of tests
- Emissions not tested: unable to be completed · 0.27% of tests
- Catalyst emissions: carbon monoxide content after 2nd fast idle exceeds manufacturer's specified limits · 0.20% of tests
- Catalyst emissions: carbon monoxide content after 2nd fast idle exceeds default limits · 0.14% of tests
- Engine oil leaks: leaking excessively from engine · 0.13% of tests
- On or after 01/07/2008: emissions exceed manufacturer's specified limit · 0.12% of tests
- Catalyst emissions: emissions likely to be affected by an exhaust leak · 0.12% of tests
Check the car you are looking at
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UK number plate or 17-character VIN
About this data. These are MOT test results, not a measure of mechanical reliability: a car's MOT outcome reflects its condition on the day, which depends on how it has been maintained and driven as well as how it was built. Figures cover class 4 vehicles (cars and light vans), normal tests only (retests excluded), outcomes pass, fail and pass after rectification at the station; vehicles at least 2.5 years old at test. Tests from 2023-01-01 to 2024-12-30. “Compared with similar cars” means compared with the national rate for cars of the same age, mileage band and fuel. Contains public sector information licensed under the Open Government Licence v3.0. Makes and models are matched to 73 Auto's vehicle catalogue; tests that can't be matched are left out rather than guessed. See how the MOT figures are produced.