The Motor Vehicles (Approval) Regulations 2001

2.  When the engine by which it is propelled is running without load at a normal idling speed, the carbon monoxide content of the exhaust emissions from the engine shall not exceed—

(a) if the vehicle has an effective date before 1st August 1986, 4.5%; or

(b) if the vehicle has an effective date on or after 1st August 1986 and not being a vehicle to which sub-paragraph (c) or (d) below applies, 3.5%; or

(c) if the vehicle is of a description mentioned in the Annex to the emissions publication and is either—

(i) a passenger car having an effective date on or after 1st August 1992; or

(ii) any other vehicle having four or more wheels and an effective date on or after 1st August 1994, the relevant percentage; or

(d) if the vehicle is not of a description mentioned in the Annex to the emissions publication and is either—

(i) a passenger car having an effective date on or after 1st August 1995; or

(ii) any other vehicle having four or more wheels and an effective date on or after 1st August 1997, 0.5%;

of the total exhaust emissions from the engine by volume.