We can take a look under the hood
Vehicle inspection is a procedure mandated by national or subnational governments in many countries, in which a vehicle is inspected to ensure that it conforms to regulations governing safety, emissions, or both. Inspection can be required at various times, e.g., periodically or on transfer of title to a vehicle. If required periodically, it is often termed periodic motor vehicle inspection; typical intervals are every two years and every year. When a vehicle passes inspection, often a sticker (inspection decal or inspection sticker) is placed on the vehicle’s windshield or registration plate to simplify later controls.
Inspection stations are places to drive inside to see if a vehicle passes inspection once a vehicle is due for inspection. Most US inspection decals/stickers display the month’s number and the year.
In some jurisdictions, proof of inspection is required before a vehicle licence or license plate can be issued or renewed. In others, once a vehicle passes inspection, an inspection decal is attached to the windshield or registration plate, and police can enforce the inspection law by seeing whether the vehicle displays an up-to-date decal.
There is some controversy over whether periodically inspecting motor vehicles is a cost-effective way to improve road traffic safety