What readiness monitors are
A vehicle from 1996 or newer runs a set of built-in self-tests on its emissions systems, such as the catalytic converter and evaporative system. Each test is called a readiness monitor. When a monitor has finished, it is marked “ready” or “complete.” A smog check on a modern vehicle reads that status through the computer.
How they get reset
Monitors reset when the computer’s memory is cleared. That can happen when:
- The battery is disconnected or replaced
- Someone clears trouble codes with a scan tool
- A repair involves reprogramming the computer
The check engine light may be off afterwards, but the monitors are not ready. A car can fail a smog check for that reason alone.
What the rules say
The California Bureau of Automotive Repair (BAR) adopted rules, effective October 1, 2025, requiring that all readiness monitors be set for a vehicle to pass. BAR says the change is being phased in gradually while it collects data, and it will announce changes in advance. In short, plan on all monitors being ready. BAR also says it will grant exceptions for vehicles that cannot meet the requirement because of a unique design issue.
How to get monitors ready
Monitors complete through normal driving or through a specific drive cycle. Every vehicle is a little different, and some need certain conditions, such as a cold start, steady highway speeds or stop-and-go city driving. A common recommendation is a week or so of ordinary driving after a battery change or repair, but the exact time varies by vehicle. Your owner’s manual or repair shop can tell you the drive cycle for your model.
Tips
- Do not disconnect the battery or clear codes right before your smog check.
- If you replaced the battery, drive your usual routes for several days before you test.
- Avoid testing with the fuel tank nearly empty or completely full. Extreme fuel levels can keep the evaporative monitor from finishing.
- If a monitor will not complete after a lot of driving, ask a repair shop to check for a fault.
Still stuck?
If your vehicle cannot complete a monitor for reasons outside your control, BAR points drivers to the state Smog Check Referee for help. See our failed smog check help page, or call 424-354-6111 and we will explain what to expect.
Sources: the California Bureau of Automotive Repair and the DMV smog check pages. Rules and fees can change, so always check your DMV renewal notice for what applies to your vehicle. This article is general information, not legal advice.
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