In most places that test emissions, a lit check engine light is an automatic fail. The station also checks the car's readiness monitors, its own self tests. If too many say not ready, often because codes were just cleared or the battery was disconnected, the car can be turned away even with no light on.
The inspector plugs into the same port under the steering wheel that an adapter uses. The computer reports whether it is turning the check engine light on, which codes are stored, and whether each readiness monitor has finished: the catalyst, the oxygen sensors, the fuel vapor system and others.
Under federal guidance that many states follow, cars from model year 2001 on may have one monitor not ready, and 1996 to 2000 cars may have two. Your state can set its own rules, so check with its motor vehicle or environmental agency.
Clearing codes or disconnecting the battery resets every monitor to not ready. Setting them again takes driving: often a few days of mixed city and highway trips with at least one cold start. Some, like the fuel vapor monitor, need specific conditions and can take longer.
A car with several monitors not ready and only a few miles since its codes were cleared may have been cleared to hide a problem. That is exactly what our Used-Car Check looks for, along with codes the seller cannot erase.
Usually not. Clearing resets the readiness monitors, and most inspections allow only one or two that are not ready.
An OBD2 adapter can read them. In the app, every scan lists each emissions monitor as ready or not ready.
No. Some states and counties test and others do not, and the rules differ. Check your state's motor vehicle or environmental agency.
Estimates only. A mechanic should confirm. If the car shows a hot temperature gauge, an oil pressure warning, smoke or a burning smell, stop as soon as it is safe.