Readiness Monitors Explained: Why Your Car Is "Not Ready" for an Emissions Test
Reviewed 2026-10-03. Checked against the sources listed below. This is general information, not a substitute for a mechanic's inspection.
Quick answer
Your car's computer runs self-tests on emission systems such as the catalytic converter, oxygen sensors and evaporative system. Each test has a readiness monitor that reports "ready" once the test has run. Clearing codes or disconnecting the battery resets them to "not ready". Emissions tests reject cars with too many monitors not ready, so after a repair you need to drive normally for a few days before testing.
What "ready" and "not ready" mean
| Status | Meaning |
|---|---|
| Ready or complete | The computer has run this self-test since the memory was last cleared |
| Not ready or incomplete | The self-test has not run yet |
| N/A | The vehicle does not have this monitor |
Ready does not mean passed. It means the test has been done. A failed test turns on the check engine light and stores a code.
Why monitors reset
- Clearing codes with a scan tool
- Disconnecting or replacing the battery
- Some repairs that require a reset
Every time this happens, the computer starts over.
How many can be not ready?
Rules depend on where the vehicle is registered. In Texas, the Department of Public Safety allows one non-continuous monitor to be not ready on 2001 and newer vehicles, and two on 1996 to 2000 vehicles. Other states and provinces set their own rules, so check your local program.
How to get monitors ready
- Drive normally for a few days with a mix of city and highway driving. Ohio EPA suggests about two or three days, longer for older vehicles. California's Smog Check program suggests driving as you normally do for about a week.
- Do not clear codes again during this time. Each reset starts the process over.
- Check with a scan tool before going for the test. Many auto parts stores can check readiness.
- Look up your vehicle's drive cycle in the owner's manual or service information if one monitor will not complete.
A tip from Ohio EPA
If possible, ask the shop not to clear the codes after a repair. If the repair was right, the check engine light turns off by itself once the monitors run, which also tells you the car is ready.
Why this matters after a converter replacement
The catalyst monitor only runs under certain driving conditions. If the codes were cleared and you test too soon after replacing a converter, the car can be rejected for not ready even though the repair worked.
