A car arrives at an emissions test with no warning light and no stored codes, and fails anyway. This catches people out constantly, and the reason is readiness monitors.
What a monitor is
The engine computer runs a set of self tests on the emissions equipment. Each one is called a monitor, and each has its own name: catalyst, evaporative system, oxygen sensor, oxygen sensor heater, exhaust gas recirculation, and several others depending on the vehicle.
Each monitor reports one of two states. Complete means the test has run since the last time the computer’s memory was cleared and it has a result. Incomplete, or not ready, means it has not run yet.
An emissions inspection checks three things: that the light is off, that no codes are stored, and that the monitors have run. A vehicle with everything working but the monitors sitting at not ready fails on the third point.
Why clearing codes causes this
When you clear trouble codes, whether with a scan tool or by disconnecting the battery, you also reset every monitor to not ready. The computer has thrown away its results along with the codes, and it has to run all the tests again from scratch.
That is the trap. Somebody clears the light the day before an inspection so the car will pass, and in doing so guarantees it will fail. The light being off is exactly what they wanted; the monitors being not ready is what they did not know about.
If you have just cleared codes, you need to drive the vehicle for a while before an inspection. How long depends on the vehicle and on which monitors are involved, but a few days of ordinary mixed driving is a reasonable expectation.
Why some monitors take so much longer than others
Monitors do not all run under the same conditions, and some are far fussier than others.
The oxygen sensor heater monitor runs on almost every cold start, so it completes quickly.
The catalyst monitor needs the engine fully warm and a period of steady cruising. Short trips around town may never satisfy it.
The evaporative monitor is the fussiest of all. It typically needs the fuel level between about a quarter and three quarters of a tank, the engine to have been off long enough to cool properly, and the outside temperature to fall within a range. That combination might not happen for days.
This is why the advice to complete a drive cycle is less useful than it sounds. There is a manufacturer specific drive cycle for most vehicles, and following it precisely does work, but for most people the honest answer is to drive normally for several days with the tank around half full and check again.
Most tests allow one incomplete monitor
Inspection rules vary, but many jurisdictions allow a small number of monitors to be incomplete and still pass, commonly one on a newer vehicle and two on an older one. The evaporative monitor is frequently the one that is permitted to be incomplete, precisely because it is so hard to run.
So it is worth checking exactly which monitors are not ready rather than assuming a fail. One incomplete evaporative monitor may well be fine. Four incomplete monitors will not be.
What to do about it
Check the monitor status before the inspection, not at it. Any generic OBD-II adapter can read readiness status, and DriveDecode shows it directly.
If monitors are not ready and you have not cleared any codes, that is worth understanding rather than driving through. A monitor that never completes often means a fault is preventing the test from running at all. The catalyst monitor cannot run without a working downstream oxygen sensor, so P0141 blocks it. The evaporative monitor cannot run with a vent valve that will not seal, so P0446 blocks it.
In that situation the monitor is not slow, it is stuck, and the fix is to repair the fault that is blocking it.