What Are Readiness Monitors and How Do I Get Them Ready for a Smog Check?

If your vehicle recently had a battery disconnected, diagnostic trouble codes cleared, or emissions repairs performed, you may hear that the readiness monitors are not ready.

Readiness monitors are self-tests built into your vehicle’s onboard diagnostic system. They check whether important emissions-control systems are operating correctly. During a California Smog Check, these monitors are reviewed as part of the OBD inspection on applicable vehicles.

What Are OBD Readiness Monitors?

Modern vehicles continuously monitor many engine and emissions systems.

Common readiness monitors include:

• Catalytic converter

• Evaporative emissions system

• Oxygen sensors

• Oxygen sensor heater

• EGR or VVT system

• Diesel particulate filter on applicable diesel vehicles

• Other emissions-related components

A monitor is usually reported as either ready/complete or not ready/incomplete.

A “ready” monitor generally means the vehicle has completed the required self-test. A “not ready” monitor means the test has not yet completed.

Why Do Readiness Monitors Reset?

Readiness monitors often reset after:

• The battery is disconnected

• The battery is replaced

• Diagnostic trouble codes are erased

• Certain emissions repairs are performed

• The vehicle’s computer loses stored diagnostic information

BAR specifically notes that readiness monitors need to be rerun after repair activity such as disconnecting the battery or replacing a defective emissions component.

Can a Car Fail Smog Because the Monitors Are Not Ready?

Yes.

California has tightened its readiness-monitor standards. Regulations effective October 1, 2025 require applicable readiness monitors to be completed before a vehicle can pass, with BAR/CARB exceptions for vehicles that cannot feasibly set particular monitors because of vehicle-specific design issues.

That means clearing trouble codes immediately before a Smog Check can create a new problem: even if the check engine light stays off, the vehicle may still have incomplete monitors.

How Do I Get My Readiness Monitors Ready?

For many vehicles, the monitors complete automatically during normal driving.

However, there is no single universal mileage or drive cycle that works for every car.

BAR explains that monitor completion can require specific operating conditions involving:

• Vehicle speed

• Engine temperature

• Engine load

• Pressure

• Driving conditions

• Time at particular speeds

Some monitors complete quickly during routine driving. Others can require very specific conditions.

How Long Should I Drive After Disconnecting the Battery?

California BAR advises motorists who recently disconnected or replaced their battery to drive the vehicle for a week or two before the Smog Check so the readiness monitors have an opportunity to reset.

That is general consumer guidance, not a guarantee that every vehicle will be ready after a particular number of days.

Some vehicles may complete their monitors sooner, while others may require a manufacturer-specific drive cycle.

What Is a Drive Cycle?

A drive cycle is a sequence of operating conditions designed to allow the vehicle’s computer to perform its emissions self-tests.

Depending on the vehicle, a drive cycle may involve combinations of:

• Cold start

• Idling

• Steady-speed driving

• Acceleration

• Deceleration

• Highway driving

• City driving

The exact procedure varies by manufacturer and model.

BAR notes that drive-cycle information may sometimes appear in the owner’s manual but is often found in manufacturer or professional repair information.

Should I Just Drive 50 or 100 Miles?

There is no guaranteed mileage.

You may hear recommendations such as 50, 100, or 200 miles, but mileage alone does not determine whether every readiness monitor completes.

The vehicle has to experience the correct operating conditions for each monitor.

A car could potentially drive many miles and still have one monitor incomplete if the conditions required for that particular self-test never occur.

Can I Clear the Codes to Make the Car Pass?

Clearing codes right before the Smog Check is usually counterproductive.

It can:

• Erase diagnostic trouble codes

• Turn the check engine light off temporarily

• Reset readiness monitors to incomplete

On newer vehicles, permanent diagnostic trouble codes can also be relevant. BAR states that permanent DTCs cannot simply be erased with a scan tool or by disconnecting the battery; the OBD system must verify that the problem has actually been corrected before the code clears.

Does the Check Engine Light Need to Be Off?

Yes.

BAR advises that a vehicle will not pass a Smog Check with the check engine light on due to an emissions-system problem.

If your check engine light is illuminated, have the underlying problem diagnosed and repaired rather than simply erasing the code.

Do All Vehicles Have the Same Readiness Rules?

No.

The inspection requirements depend on factors such as:

• Model year

• Fuel type

• Vehicle design

• The particular monitor involved

BAR maintains vehicle-specific OBD guidance for vehicles with known difficulties completing certain monitors. In some cases, the inspection system is programmed to accommodate a known vehicle-design issue.

What About Diesel Vehicles?

Diesel vehicles also use readiness monitors when subject to an OBD Smog Check.

California applies specific readiness standards based on model year and monitor type. BAR has also adopted exceptions for certain vehicle designs where a monitor cannot reasonably be completed.

If you have a diesel vehicle and are unsure whether it is ready, it is better to verify the monitor status before the official inspection.

Can a Smog Shop Tell Me Whether My Monitors Are Ready?

The vehicle’s readiness status can be checked through its onboard diagnostic system.

If you recently had repairs completed, replaced your battery, or cleared codes, checking the monitor status before an official Smog Check may save you an unnecessary failed inspection.

What Should I Do Before My Smog Check?

Before bringing the vehicle in:

1. Make sure the check engine light is not on.

2. Avoid clearing diagnostic codes immediately before the test.

3. If the battery was recently disconnected or replaced, allow time for the monitors to complete.

4. Drive the vehicle normally.

5. If monitors remain incomplete, consult vehicle-specific drive-cycle information or have the vehicle diagnosed.

6. Bring the vehicle in once the required monitors are ready.

Frequently Asked Questions

What does “monitor not ready” mean?

It means the vehicle’s onboard diagnostic system has not yet completed one of its emissions-related self-tests.

Will disconnecting my battery reset the monitors?

Yes, it can. BAR specifically advises motorists to drive the vehicle after a battery disconnect or replacement so the monitors can run again.

How many miles do I need to drive?

There is no universal mileage requirement. Completion depends on the vehicle and the operating conditions required for each monitor.

Can I pass with one monitor incomplete?

California’s updated regulations require all monitors that can feasibly be set to be complete, with BAR-approved exceptions for certain vehicle-specific design issues.

Do I need an appointment at Smog Tech Pros?

No. Walk-ins are welcome.

Need a Smog Check in Rancho Cordova?

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Rancho Cordova, CA 95670
916-638-4700

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