Emissions lambda reading at the MOT: which cars fail on it most
1.9% of failed initial MOT tests in 2025 recorded this defect: 176,447 of 9,185,328. By age, it was most common on vehicles 20 years old and over (4.0%) and least common on vehicles 3 to 5 years old (0.4%).
It is one of the 10 most common MOT failures on 24 of the 332 car models we cover. Against vehicles of the same age, it was most common on the Smart Forfour, the Peugeot 208 and the Porsche Boxster. On the Smart Forfour, 11.5% of failed tests had it, against 1.2% for all vehicles of the same age.
The DVSA records it as “Catalyst emissions lambda reading after 2nd fast idle outside specified limits”. A failed test can have several defects.
What the MOT checks
Petrol engines with a catalytic converter have their exhaust gas measured at fast idle. The lambda reading is outside the default limits or the range the manufacturer specifies.
The MOT inspection manual lists “Lambda coefficient outside the default limits or the range specified by the manufacturer” as a major defect, so the vehicle fails.
Car models where it is most common
| # | Car | Share of failed tests | Against same age |
|---|---|---|---|
| 1 | Smart Forfour5,678 failed tests | 11.5%651 tests | 9.3×all: 1.2% |
| 2 | Peugeot 20864,789 failed tests | 8.1%5,256 tests | 6.1×all: 1.3% |
| 3 | Porsche Boxster4,873 failed tests | 18.5%902 tests | 4.7×all: 3.9% |
| 4 | Fiat Grande Punto3,500 failed tests | 12.5%437 tests | 4.7×all: 2.7% |
| 5 | Renault Twingo7,985 failed tests | 9.0%718 tests | 4.5×all: 2.0% |
| 6 | Ford EcoSport25,438 failed tests | 4.6%1,166 tests | 4.5×all: 1.0% |
| 7 | Fiat Punto20,760 failed tests | 10.0%2,068 tests | 4.1×all: 2.4% |
| 8 | Skoda Citigo11,430 failed tests | 5.8%667 tests | 4.1×all: 1.4% |
| 9 | Porsche Cayman1,044 failed tests | 9.4%98 tests | 4.0×all: 2.3% |
| 10 | SEAT Mii4,453 failed tests | 5.8%259 tests | 4.0×all: 1.5% |
| 11 | Volkswagen Fox11,461 failed tests | 10.7%1,225 tests | 4.0×all: 2.7% |
| 12 | Volkswagen Up26,493 failed tests | 5.3%1,416 tests | 3.7×all: 1.4% |
| 13 | Alfa Romeo Mito6,404 failed tests | 7.3%466 tests | 3.7×all: 2.0% |
| 14 | Porsche 9114,515 failed tests | 10.9%491 tests | 3.5×all: 3.1% |
| 15 | Subaru Impreza2,185 failed tests | 11.4%248 tests | 3.3×all: 3.4% |
Only models where this defect is one of their 10 most common MOT failures, with at least 50 failed tests, have a figure. So a model that isn't listed can still have it, just less often than its other faults. Vans, pick-ups and taxis are left out, as in our rankings. × is the model's share divided by the share for all vehicles of the same age.
How common it is by car age
- 3 to 5 years, first used 2020 to 20220.4% of failed tests, against 1.9% for all ages2,605 failed tests
- 6 to 9 years, first used 2016 to 20191.1% of failed tests, against 1.9% for all ages22,631 failed tests
- 10 to 14 years, first used 2011 to 20151.7% of failed tests, against 1.9% for all ages56,877 failed tests
- 15 to 19 years, first used 2006 to 20102.8% of failed tests, against 1.9% for all ages61,588 failed tests
- 20 years and over, first used 1985 to 20054.0% of failed tests, against 1.9% for all ages32,717 failed tests
Not shown, with fewer than 50 failed tests with this defect: vehicles under 3 years old.
All-vehicle figures cover vehicles first used from 1985 to 2024: 9,185,328 of the 9,193,164 failed initial tests in 2025.
How these figures are counted
These are failures recorded at initial MOT tests in 2025. Retests are not counted. A defect found on more than one wheel, lamp or joint at the same test counts once. "Same age" is the defect's share of failed tests for all vehicles first used in the same years, weighted by the model's own failed tests in each year.
An MOT checks safety and emissions on the day of the test. These figures show where a defect was recorded, not why, and not what a repair costs. Figures: DVSA anonymised MOT tests and results, 2025. Contains public sector information licensed under the Open Government Licence v3.0. What the MOT checks: MOT inspection manual: cars and passenger vehicles, GOV.UK, read 1 Nov 2026.
How we choose and rank these defectsOther noise, emissions, leaks faults
Share of all failed initial MOT tests in 2025.