Use the inspection result, monitor status, and code family to separate an active emissions failure from a car that simply never completed its self-tests after repair or code clearing.
A failed emissions result can mean two very different things. The car may have an active fault code that the test station sees immediately, or it may have too many monitors listed as not ready even though the MIL is off. Treating those as the same problem is how people replace parts for a drive-cycle issue or chase drive cycles when the converter, air-injection, oxygen-sensor, or EGR system still has a live fault.
Readiness gets erased by recent code clearing, battery disconnects, and some module resets. A repaired vehicle can still fail inspection if the EVAP, catalyst, oxygen-sensor, oxygen-heater, EGR, or secondary-air monitor simply has not completed under the right fuel level, temperature, speed, and soak conditions yet. The first job is to see which monitors are incomplete, not to assume the last replaced part was wrong.
P0420 and P0430 push you toward catalyst efficiency and the upstream problems that damage converters. P0410 through P0418 point toward secondary-air operation, which can block readiness even when the car feels fine. Upstream and downstream oxygen-sensor codes like P0130, P0133, P0140, P0150, P0153, and P0154 distort fuel-control or monitor trust directly. P0401 and P0402 matter because EGR flow problems can keep the EGR monitor from completing honestly. EVAP leak and monitor-hardware codes remain their own branch, but they should no longer be mistaken for the whole emissions story.
Many cars with catalyst, secondary-air, rear-O2, or EVAP monitor issues can drive almost normally. That is exactly why emissions diagnosis gets sloppy. The engine can feel fine while the monitor logic still refuses to certify the repair. A smooth test drive is helpful, but it is not proof that every required monitor has completed.
First read stored, pending, and permanent codes plus current monitor status. Second decide whether you are dealing with an active fault or an incomplete monitor set. Third fix any live fault that obviously blocks readiness. Fourth confirm the specific monitor prerequisites for fuel level, cold soak, ambient temperature, and cruise or idle portions of the drive cycle. Fifth only send the car back for inspection after the required monitors have actually completed on the scan tool.
A general failed-emissions result is a routing problem before it is a parts problem. Split active faults from incomplete monitors, then split the incomplete monitors by system: catalyst, oxygen sensor, secondary air, EGR, or EVAP. Once that architecture is clear, the next diagnostic step becomes much more honest.