OBD2 Readiness Monitors Explained: What Not Ready Means
Quick answer: An OBD2 readiness monitor is a self-check of an emissions-related system. Ready means the check has completed; Not Ready means it has not finished yet. After clearing trouble codes or disconnecting the battery, some monitors may reset to Not Ready. That alone does not tell you a part is bad, but it may affect an emissions inspection.
You cleared a check engine light, drove the car, and now your scanner says "Catalyst: Not Ready" or "EVAP: Incomplete." What should you do next? This guide explains the message, how to read the status, and when driving is enough versus when you should investigate a fault.
Readiness is not the same as a pending, stored, or permanent trouble code. A code reports a detected fault; a readiness status tells you whether an emissions-related test ran.
Ready, Not Ready, and Not Supported: The Three Messages
- Ready / Complete / OK: The monitor completed its required check since the last relevant reset. It does not guarantee that every part on the car is perfect.
- Not Ready / Incomplete / INC: The check has not finished under the required conditions. It is not, by itself, a failed component diagnosis.
- Not Supported / N/A / N/S: Your vehicle does not support or require that specific monitor. You do not have to make an unsupported monitor "ready."
Example: After a battery replacement, Misfire may show Ready while Catalyst and EVAP show Not Ready. That can be normal while the car completes its checks. If a trouble code or warning returns, diagnose it instead of assuming more driving will fix it.
How to Check I/M Readiness With an OBD2 Scanner
- Park safely. Switch off unnecessary accessories, locate the OBD2 port, and connect the scanner. Follow the scanner's instructions for ignition position.
- Read your trouble codes first. Save stored and pending codes plus freeze-frame information, if available. Do not choose "Erase" or "Clear."
- Open the emissions menu. Look for I/M Readiness, Monitors, Emissions, or Inspection Status. Menu names vary.
- Record the status. Note which monitors say Ready, Not Ready, or Not Supported. Take a screenshot or photo if that is easier.
- Compare after normal driving or a repair. Recheck the same screen without erasing codes again.
A basic code reader may show only an overall readiness result; a more capable scanner or app may list the individual monitors. If you need the general scan sequence first, see how to use an OBD2 scanner.
Why Are My Readiness Monitors Not Ready?
Common explanations include:
- Codes were recently cleared. Clearing codes normally resets applicable readiness checks, even if the check engine light is off.
- The battery lost power. A disconnected or dead battery can reset readiness information.
- The car has not met the test conditions. Some checks need a certain engine temperature, fuel level, driving pattern, or other vehicle-specific conditions.
- A fault prevents completion. An active or pending trouble code, or another system problem, may stop a particular test from running successfully.
- A vehicle-specific issue exists. Some models need a particular procedure or have service bulletins about hard-to-complete monitors.
Important: Erasing codes repeatedly can restart the process. It can also remove clues a technician needs. The right next step depends on whether a fault is present, not simply on how many miles you have driven.
What to Do When a Monitor Says Not Ready
- Check for safety warnings. If the check engine light is flashing, the engine is misfiring badly, or the vehicle has serious driveability symptoms, stop treating this as an inspection-only issue and get the problem evaluated.
- Save any codes. If the check engine light is on or codes are present, investigate and repair the cause before trying to complete monitors. Do not use code clearing to hide the warning.
- Think about recent work. Did you replace the battery, repair an emissions part, or press Clear Codes? That explains why several monitors might still be incomplete.
- Drive normally if the car is operating safely. Some monitors complete during ordinary use. Allow the engine to reach normal operating temperature when appropriate; avoid unsafe driving maneuvers just to trigger a test.
- Check the vehicle-specific drive cycle. If one monitor stays incomplete, consult the owner's manual, manufacturer service instructions, or a qualified technician. There is no universal speed, mileage, or fuel-level recipe.
- Scan again before inspection. Confirm which monitors have completed and check the official rules for your state and vehicle type.
If the light goes out and comes back, our guide to an intermittent check engine light explains why you should still save the code history.
Continuous vs Non-Continuous Monitors
Continuous monitors generally watch for misfires, fuel-system issues, and comprehensive component problems while the engine runs. Non-continuous monitors need particular conditions before their tests can finish. The set of supported monitors varies by vehicle and powertrain.
Examples of non-continuous tests you may see include:
- Catalyst: checks aspects of catalytic converter performance.
- EVAP: checks parts of the fuel-vapor control system.
- Oxygen sensor and heater: checks certain sensor and heater operations.
- EGR or secondary air: appears only on vehicles with the relevant emissions system.
When a P0420 catalyst-efficiency code or an EVAP code such as P0440 is involved, work on the underlying fault first. Readiness is evidence that a self-check completed, not a substitute for diagnosis.
Why Won't the Catalyst or EVAP Monitor Become Ready?
Different monitors have different prerequisites. For example, an EVAP check may need a vehicle-specific fuel level and ambient conditions. A catalyst check may depend on engine temperature and other systems working properly. If the car never meets those conditions, or a related fault is present, the monitor may remain incomplete.
The safe approach is to look up the procedure for your exact model year, make, model, engine, and emissions configuration. Avoid internet instructions that promise to set every monitor in "15 minutes" with the same highway routine. Never coast, brake, or accelerate in a way that creates a traffic hazard to complete a scanner test.
When to ask a shop: If the same monitor remains Not Ready after appropriate driving, especially with a pending/stored code, ask the shop to check the related fault and whether a service bulletin or manufacturer-specific drive cycle applies.
Can You Pass an Emissions Test With a Monitor Not Ready?
Sometimes, but do not assume it. Emissions inspection requirements differ by state, model year, fuel type, and vehicle. A check engine light commanded on can also cause an inspection failure. The fact that an OBD2 scanner says Not Ready does not tell you whether your particular state allows an exception.
For example, the California Bureau of Automotive Repair OBD test reference publishes its current monitor pass/fail standards and special vehicle exceptions. California has also announced a phased approach to changing readiness requirements; check the current reference rather than relying on an older rule quoted in a forum. Your local inspection agency is the final source for your location.
Before booking a retest: Write down the monitors that remain incomplete, check the car for trouble codes, and confirm the applicable inspection rule. Avoid clearing the codes right before the test; that typically resets the checks you are trying to complete.
Readiness vs Pending, Stored, and Permanent Codes
These terms answer different questions. A readiness monitor asks, "Has the car run this emissions self-check?" A diagnostic trouble code asks, "Has the car detected a particular fault?"
- Not Ready with no codes: The self-check may simply need the correct operating conditions after a reset.
- Not Ready with a fault code: Diagnose the underlying problem; driving alone may not resolve the situation.
- Ready with a stored code: A completed monitor status does not cancel a fault or prove the vehicle will pass inspection.
- Permanent code present: Its removal follows separate vehicle self-verification rules; do not confuse that with the individual readiness flags.
For code statuses in more detail, read Pending vs Stored vs Permanent Codes.
Example: The Check Engine Light Is Off After a Battery Replacement
You replaced a dead battery, and the car now runs normally with no warning light. Your scanner shows Misfire: Ready, Catalyst: Not Ready, and EVAP: Not Ready. This can happen because the battery interruption erased some prior self-test results.
Your next move: Save the current code and readiness scan, drive normally if the car is safe, then recheck. If a trouble code appears or the same monitor never finishes under its required conditions, use vehicle-specific service information or get a diagnosis. Do not keep resetting it.
Frequently Asked Questions
Does Not Ready mean my car failed a test?
Not necessarily. It means that particular emissions self-test has not finished. Check for trouble codes, complete the vehicle-specific driving conditions, and scan again.
Will clearing codes make readiness monitors ready?
No. Clearing trouble codes usually resets applicable readiness monitors to Not Ready. Save diagnostic information first and avoid clearing codes just before an emissions inspection.
How many miles does it take for readiness monitors to become ready?
There is no universal mileage. Each vehicle and monitor needs particular operating conditions. Use your vehicle service information rather than relying on one distance or highway-speed formula.
Can I pass a smog test with one monitor Not Ready?
It depends on your state, vehicle, fuel type, model year, and current inspection rules. Check the official requirements for your location before the test.
What does Not Supported mean on an OBD2 scanner?
Not Supported or N/A means that the vehicle does not use or report that particular emissions monitor. It is not the same as Not Ready.
Where This Guidance Comes From
Readiness labels and the distinction from trouble codes follow the Oregon Department of Environmental Quality's OBD FAQ. Inspection and vehicle-specific exceptions are described in the California BAR OBD test reference and its 2025 update. Specific drive cycles should come from the vehicle manufacturer.