OBD2 readiness monitors explained
Readiness monitors are the car's own emission self tests. A scanner shows each one as ready, not ready or not supported. Not ready usually means the test has not run yet, not that something is broken.
What is a readiness monitor?
The OBD system checks up to 11 emission related systems with its own self tests. Each test reports a status: ready means it has run, not ready means it has not run yet since the memory was last cleared, and not supported or not applicable means the car does not have that system.
Inspection programs read these statuses to decide whether the OBD system has had enough time to check the car. A not ready monitor is not itself a fault code.
Continuous and non-continuous monitors
Three monitors run continuously while the engine runs. The other eight only run when specific conditions are met, and not every car has all of them.
| Monitor | Type |
|---|---|
| Misfire | Continuous |
| Fuel system | Continuous |
| Comprehensive components | Continuous |
| Catalyst | Non-continuous |
| Heated catalyst | Non-continuous |
| Evaporative system (EVAP) | Non-continuous |
| Oxygen sensor | Non-continuous |
| Oxygen sensor heater | Non-continuous |
| Exhaust gas recirculation (EGR) | Non-continuous |
| Secondary air | Non-continuous |
| Air conditioning | Non-continuous |
The conditions that let each non-continuous monitor run, such as temperature, speed and engine load, are set by each manufacturer, so the time needed varies by car.
Why monitors show not ready
- The codes were cleared with a scan tool. Clearing resets the monitors.
- The battery was disconnected, went flat or was replaced.
- The car has not yet been driven under the conditions a monitor needs, for example only short trips.
- A repair was just done and the codes were cleared afterwards.
How to get monitors to ready
In most cases, drive normally. California guidance suggests about a week of normal driving after codes are cleared or the battery is disconnected. Mix in some longer drives at steady speed rather than only short trips.
Some monitors need a specific drive cycle. Your owner's manual may describe it, and a technician or dealer can look it up in the manufacturer's service information. Recheck with your scanner before the inspection.
How many not ready monitors are allowed at an inspection?
Rules depend on where the car is tested. Two published examples:
| Program | Model years | Not ready monitors allowed |
|---|---|---|
| US EPA guidance (used for example in Texas) | 1996 to 2000 | Up to 2 non-continuous |
| US EPA guidance (used for example in Texas) | 2001 and newer | Up to 1 non-continuous |
| California Smog Check, gasoline | 1996 to 1999 | Any one |
| California Smog Check, gasoline | 2000 and newer | Only the EVAP monitor |
A check engine light that is commanded on still fails the OBD test regardless of readiness. Check your local program's current rules before you go.
Checking readiness with OBD Master Pro
Connect your ELM327 class adapter, open the emission readiness check and note which monitors are not ready. If you just cleared codes after a repair, drive for a few days and check again. You can also compare the result with any stored or pending codes, since a pending code can explain why a monitor will not complete.
Try it on your own car
OBD Master Pro turns your Android phone and a generic ELM327 adapter into a code reader: read and clear codes, view freeze frame and live data, and check emissions readiness.
Keep reading
Related guides: Check engine light on: what to do · How to read an OBD2 code · Stored, pending and permanent codes · ELM327 adapter not connecting · How to clear the check engine light · What is fuel trim · OBD2 live data and PIDs · OBD2 port pinout · What is a U code? · OBD2 glossary
Common codes: P0171 System Too Lean · P0420 Catalyst Efficiency · P0300 Random Misfire · P0455 EVAP Large Leak
All OBD2 guides · OBD2 by car brand · OBD2 trouble code guides
Sources
- U.S. EPA: Performing Onboard Diagnostic System Checks as Part of a Vehicle Inspection and Maintenance Program (EPA420-R-01-015)
- Texas Department of Public Safety: Ready or Not (OBD II readiness monitors)
- California Bureau of Automotive Repair: On-Board Diagnostic Test Reference
- California Bureau of Automotive Repair: Permanent Diagnostic Trouble Codes, workshop presentation (2018)
- California Air Resources Board (CARB): On-Board Diagnostic II (OBD II) Systems fact sheet
- California Bureau of Automotive Repair: Smog Check Program FAQ
- OBD Master Pro on Google Play (app features, supported connections and protocols)
Published . Updated . General information only. Details generally vary by model, year and market, so always follow the owner's manual and service information for your vehicle.