If your car is classified as Euro 5 on the V5/C registration document then to make it ULEZ-compliant by reaching Euro 6 you would not only have to modify the vehicle extensively to meet this standard, you would also need to pay a substantial amount of money for it to be tested. The ANPR cameras that enforce the ULEZ scan numberplates and then match these agains the DVLA database to check the emissions standard of each individual vehicle against its Type Approval rating, which is set when the vehicle is first made available on the market.

To meet the Euro 6 standard you would need to add a Selective Catalytic Reduction system to your car, which means adding an AdBlue tank, pump, injectors, sensors and an electronic control unit, and as well of the cost of these parts you would also need to pay for them to be fitted, programmed into the vehicle and tested, all of which would cost several thousand pounds. Then you would need the vehicle to pass the Portable Emissions Measurement System test, and as this is designed for commercial vehicles rather than passenger cars, the cost of this would also run into several thousand pounds. The Energy Saving Trust, which operates the Clean Vehicle Retrofit Accreditation Scheme, has stated that there are no CVRAS-approved retrofit systems for Euro 5 diesel passenger cars.