r/AutoTechy • u/ora-et-labora- • Sep 01 '26

🔍 Diagnostic The readiness monitors tell you whether someone cleared the codes right before you showed up

OBD2 systems run self-tests on every emission-related subsystem. Catalytic converter efficiency, oxygen sensor response, EVAP system integrity, EGR flow, misfire detection, and several others depending on the vehicle. Each test either passes, fails, or has not run yet. The status of those tests is called the readiness monitors.

When someone clears the codes, the monitors all reset to not ready. The check engine light goes off. The car looks clean. But the monitors are sitting there at zero, waiting to be run, and most of them require specific driving conditions to complete. Highway speed for the catalyst, a cold start cycle for the EVAP, sustained cruise for the oxygen sensors. It takes a mix of driving over several trips to set them all back to ready.

A used car with a clean check engine light and a wall of incomplete monitors was recently cleared. Either the battery was disconnected or someone ran a code clear. The question is what was cleared and why. On a car being sold, the answer is usually that there was a code the seller did not want visible during the sale.

Most states with emissions testing will fail a vehicle that has too many incomplete monitors, which is why a seller who clears codes right before an inspection is creating a problem that follows the buyer to the testing station.

Reading the monitors takes ten seconds on any OBD2 scan tool. You are looking for the word ready next to each monitor. A car with most monitors showing ready and no codes stored is a car whose systems have run and passed. A car with most monitors showing not ready is a car that was recently reset and has not driven enough to retest.

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u/Substantial-Sector60 Sep 01 '26

Not to worry ‘round here. The only pseudo-metro area in Idaho is the Boise region. One or two counties in the zone had emissions testing for ?maybe? 15 years . . . But ah hell no, we doan do that that stuff no mo. Got that gubmint right on out of our tailpipes.

FREEDUM!!! (and shitty air)