It is based on the effective date of registration of your donor vehicles engine and then taken back to the 1st of January of that year. It is therefore important that you keep hold or your donor vehicles V5.
If you havn’t got a V5 then evidence from the vehicle manufacturer will be needed to confirm the manufactured date (can be determined from the engine number)
If the age of the engine cannot be determined then the effective date will be 1st August 1997 for petrol engines
To see the different emission levels for different age engines, see the emission levels article