Unlike passenger vehicles, which require biennial smog checks in California, heavy-duty diesel trucks are not currently required to undergo regular emissions testing. That might change with the introduction of a bill in the state Senate.
Colo. Initiatives Support Transition to Zero Emission Vehicles
Colorado Gov. Jared Polis has signed an executive order designed to support a transition to zero-emission vehicles and help the state reach 100% renewable electricity by 2040.