http://www.legislation.gov.uk/id/uksi/2001/25

SCHEDULE 3APPROVAL REQUIREMENTS FOR RELEVANT VEHICLES

4.

If the vehicle is one to which paragraph 2(c) or (d) applies and the engine by which it is propelled is run without a load at a fast idling speed—

(a)

the carbon monoxide content of the exhaust emissions from the engine shall not exceed 0.3% of the total exhaust emissions from the engine by volume;

(b)

the hydrocarbon content of the exhaust emissions from the engine shall not exceed 0.02% of the total exhaust emissions from the engine by volume; and

(c)

the lambda value shall be between the relevant limits.