Emissions of carbon monoxide and hydrocarbons from cars with petrol engines are measured as part of the national car test; nitrogen oxide emissions are not measured.
The applicable emission limits are as follows: For cars first registered before 1 October 1986, the carbon monoxide content must not exceed 4.5% at idling speed and the hydrocarbon content must not exceed 1,000 parts per million at idling speed.
For cars first registered between 1 October 1986 and 31 December 1993, the carbon monoxide content must not exceed 3.5% at idling speed, and the hydrocarbon content must not exceed 750 parts per million at idling speed.
For cars first registered on or after 1 January 1994, the carbon monoxide content must not exceed 0.5% at idling speed., and measured at 2,500 engine revolutions per minute, rpm, or at an engine speed specified by the vehicle manufacturer must not exceed 0.3%. The hydrocarbon limit for these cars is 200 parts per million at 2,500 rpm, or at an engine speed specified by the manufacturer.
In general, cars which were first registered on or after 1 January 1994 were fitted with catalytic equipment to comply with EU entry into service requirements. Such catalytic equipment must be in good working condition for the cars in question to comply with the relevant national car test emission limits.