MOT performance

MOT failure “Catalyst emissions: carbon monoxide content at idle exceeds default limits”: how common it is and which cars get it

“Catalyst emissions: carbon monoxide content at idle exceeds default limits” (major defect, noise, emissions and leaks) was recorded as a failure on 0.06% of UK MOT tests in 2023–2024: 39,264 tests, the number 121 most common failure item. It is 7.0 times as common at 10 years old (0.03%) as at 3 (0.01%). It is most common at 20 years old (0.25% of tests).

39,264 tests with this failure, 2023–2024

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At a glance

“Catalyst emissions: carbon monoxide content at idle exceeds default limits” at a glance
Figure	Value	Basis
Share of MOT tests	0.06%	39,264 tests, 2023–2024
Rank among failure items	#121	by number of tests
Most common at	20 years old	0.25% of tests at that age
Models compared	163	models with enough tests

How it changes with age

Share of MOT tests failing on “Catalyst emissions: carbon monoxide content at idle exceeds default limits” by age
Age at test	Share of tests
3 years	0.01%
5 years	0.01%
7 years	0.03%
10 years	0.03%
12 years	0.04%
15 years	0.14%
18 years	0.18%
20 years	0.25%

By fuel

Petrol: 0.11% 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.

Models failing most often on “Catalyst emissions: carbon monoxide content at idle exceeds default limits”
Model	Tests	Failed on it	vs similar cars
Skoda Citigo	104,908	0.25%	+347%
Peugeot 308	274,758	0.12%	+321%
Volkswagen Up	247,835	0.21%	+288%
Audi Q3	250,413	0.05%	+267%
Peugeot 106	11,071	0.99%	+170%
Peugeot 208	402,103	0.10%	+164%
MG Mgf	12,064	1.18%	+155%
Audi A1	431,497	0.09%	+153%
Volkswagen Beetle	91,710	0.38%	+142%
Kia Sportage	531,226	0.04%	+139%

Models that fail on it least often

Models failing least often on “Catalyst emissions: carbon monoxide content at idle exceeds default limits”
Model	Tests	Failed on it	vs similar cars
Suzuki Splash	35,745	0.00%	-97%
Mercedes-Benz B-Class	145,990	0.00%	-94%
Toyota Auris	311,459	0.01%	-87%
Toyota iQ	34,119	0.02%	-86%
Vauxhall Viva	76,738	0.01%	-86%
Vauxhall Agila	73,056	0.02%	-85%
Mercedes-Benz SL	43,454	0.05%	-83%
Bentley Continental	29,070	0.03%	-82%
Saab 9-3	82,791	0.03%	-80%
Renault Captur	300,538	0.00%	-80%

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

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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.