The Government has enhanced a year-old scheme that encourages owners to replace their aging and pollutive diesel commercial vehicles.

From March 12, owners of such vehicles registered before 2001 will get a sweetener if they switch to models that comply at least with the Euro 5 emission standards.

On top of being able to transfer the unused portion of their vehicle's certificate of entitlement (COE) to a new vehicle, owners will get bigger COE bonuses. Light vehicles will get a 20 per cent bonus, up from 10 per cent originally, and heavy vehicles will get a 100 per cent bonus, up from 30 per cent.

The bigger incentive for the latter is to reflect the higher cost of such vehicles.

With the enhancement, an owner of a big truck could pay just $5,000 for COE on his new vehicle, instead of $22,500 previously.

As the enhanced incentives are significant, owners who renew will now have to pay a minimum of 10 per cent of a vehicle's prevailing quota premium.

The scheme was to end at the end of April this year, but has now been extended to end-April 2016.