Readiness monitors and your California smog check

What a readiness monitor is and why it can fail you

Your car tests itself all the time. Each self test is called a monitor. A monitor runs only when driving conditions are right. When it finishes, it sets a flag that says "ready." When the flag is not set, the car says "not ready."

Now the key point for a smog check. If the battery is disconnected, replaced, or if someone clears the codes, the flags reset. The car has to drive through its self tests again. A smog check done too soon can fail only because the monitors are not ready. The car might be perfectly healthy.

This is the most common reason that clean cars fail. In 2025, readiness failures hit 4.11% of all initial tests on 2000 and newer cars. That is about 430,000 cars. It also made up roughly 53% of every failure on that kind of test. About half of all OBD style failures are "not ready" results.

How many monitors can be incomplete

A car does not have to have every monitor done to pass today. The state allows some to be incomplete. Which ones depends on the car.

For a typical 2000 and newer gas car, only the evaporative system monitor may be incomplete. That monitor checks the fuel vapor system. It is slow and picky about when it will run, so BAR lets it slide for now. If any other monitor says "not ready," the car fails.

The misfire, fuel and comprehensive component monitors are ignored on gas cars. They never count against you.

The October 2025 rule, and what it means for you now

The rules are changing. California amended section 3340.42.2 of its smog check regulations. Under the new text, all readiness monitors must be complete to pass. The rule took effect on October 1, 2025.

Here is what BAR has said about the rollout. The current pass and fail criteria "will remain unchanged while BAR collects and analyzes Smog Check data." BAR will phase in changes by vehicle, once data show that monitors on those vehicles can set. It will announce changes well in advance. As of late October 2026, BAR's OBD test reference still lists the old allowances.

BAR looked at roadside checks from 2023 and 2024. It found over 90% of vehicles on the road had all of their monitors set, including the evaporative monitor. That is the case BAR makes for the tighter rule.

What should you do with this? Plan as if complete monitors will be required one day. If your car's evaporative monitor never completes, tell a repair shop. And watch BAR's news page, because the date has not been announced.

Which cars fail on readiness most

Look at the pattern. The 2000 model year failed on readiness 10.8% of the time. That is more than 5 times the rate for model year 2017, at 1.9%. Older cars have more worn sensors and take longer to set monitors. Model years 2019 to 2025 come in higher than 2017. The data do not say why.

Across all years, the readiness fail rate slipped from 4.80% in 2019 to 4.11% in 2025. It dropped in 2026, to between 3.87% and 4.14% each month. The monthly numbers are below.

How long to drive before a smog check

There is no magic number of miles. BAR says one to two weeks of driving after a battery disconnect or replacement. In its rulemaking file, BAR says two weeks is the time in which most properly working vehicles set the monitors they need. So aim for two weeks of normal use, with a mix of short and longer trips.

1. Drive normally. Let the car run through its self tests on its own.
2. Avoid resets. Do not disconnect the battery. Do not clear codes before the test.
3. Give it two weeks. That is the time BAR cites for most good cars.
4. Test. If you pass, the station sends your certificate to DMV.

Your DMV notice has a due date. A test can be done up to 90 days before it is required. So if your battery died last week, you may have time to wait.

If you must test soon and the monitors are not ready, you will get a "not ready" fail. That is a fail on the record. Learn more about what to do next in our failed smog check guide.

Plan your test window

You can use the 90 day window to your advantage. A smog check can be done any time within 90 days before it is required for your registration renewal. That is a long runway. Say your renewal is due December 1. You could test as early as early September. If your battery dies in October, you still have weeks to drive before you must test.

Sellers face a tighter clock. A certificate is valid for 90 days, and a seller has to provide one at sale. So if you plan to sell and you just replaced the battery, wait. Drive two weeks. Then test. The certificate then covers the whole sale period.

Dealers have longer. A dealer certificate is good for two years or until the car is sold and registered.

Common myths about readiness

• Myth. "A scan tool can make the monitors ready." Fact. Clearing codes does the opposite. It resets them.
• Myth. "If the light is off, I will pass." Fact. A car with no light can still fail for not ready monitors. About 4 in 100 tests in 2025 failed this way.
• Myth. "A new car cannot be not ready." Fact. The 2025 model year failed on readiness at 4.6%, more than the 2017 model year.
• Myth. "The new rule already makes every monitor required." Fact. The rule is in force, but BAR says pass and fail criteria stay unchanged while it collects data.

Why BAR wants every monitor done

A monitor that never sets can hide a fault. A car can have a failing part and still show a dark dash light if the test that would catch it has never run. That is the logic of the rule. BAR cites on road data. In its 2023 and 2024 roadside checks, over 90% of cars had all of their monitors set. If most cars on the road can set them all, BAR argues that the test can ask the same of a car at the smog station.

The industry has pushed back. In public comments on the rule, one smog technician wrote that 90% of the cars he tests do not have the evaporative monitor complete. BAR has not published a date for when the phased rollout will bite, and it has said it will give notice well ahead.

For you, the plan is simple. Treat a complete set of monitors as the goal. Drive the car normally. Avoid battery work right before a test. If a monitor will not set after weeks of driving, fix the cause.

When the monitors will not set

Sometimes the car is the problem. Two weeks pass and a monitor still says "not ready." That points to a fault. Often a code is stored. A shop can read the cause. Common problem areas on failing cars include the evaporative, EGR and catalyst systems.

Know that CAP repair help does not cover "completing OBD drive cycles." The money is for fixing parts. Learn how it works in the CAP guide.

If you cannot get a monitor to set, the Smog Check Referee is the place for OBD monitor issues. The Referee did 262 of those inspections in 2025. You reach the Referee at (800) 622-7733. Everyone starts with a virtual appointment. If you miss the call or do not show, the file locks for 60 days.

Diesels are stricter

Diesel cars from 1998 to 2006 can have no monitors incomplete. Newer diesels may have the particulate filter and NMHC catalyst monitors incomplete, but no others. Light duty diesels at 14,000 lbs or less follow the smog check rules in this guide. Heavier trucks follow CARB's Clean Truck Check instead.