New OBD readiness monitor regulations explained
To strengthen the effectiveness of the California Smog Check Program, the Bureau of Automotive Repair (BAR) recently adopted regulations governing on-board diagnostic II (OBD II) readiness monitors. Effective October 1, 2025, these regulations require that all readiness monitors must be set for a vehicle to pass a Smog Check inspection. In cases where a vehicle cannot meet this requirement due to unique design issues, BAR—in consultation with the California Air Resources Board (CARB)—will grant an exception. Implementation of the updated standards will be phased in gradually to ensure a practical, data-driven approach for the Smog Check industry and consumers.
The role of OBD II readiness monitors in a Smog Check inspection
OBD II data and readiness monitors play a key role in Smog Check inspections. OBD II is a standardized, computer-based system required on all 1996 and newer gasoline-powered passenger vehicles and light-duty trucks, and on all 1998 and newer diesel vehicles. It monitors engine and emissions controls, stores diagnostic trouble codes (DTCs) when malfunctions occur, and illuminates the malfunction indicator light (MIL) to alert the driver of the vehicle.
Readiness monitors, part of the OBD II system, indicate whether emissions-related diagnostic self-tests have been completed. When emissions repairs are made to address a DTC, a vehicle must then complete the applicable readiness monitor(s)—either through normal driving or by a specified drive cycle—so the OBD II system can determine whether the malfunction was fixed. Once all applicable monitors are set and the system shows “ready,” the MIL should no longer be illuminated, confirming the emissions controls are functioning properly. During a Smog Check, the vehicle's readiness monitors are reviewed to determine if they are set.
Challenges in setting readiness monitors
Some readiness monitors require considerably more time or specific driving conditions to set. This can create challenges, especially after repairs or a battery disconnect. Historically, BAR has allowed across-the-board exemptions for certain hard-to-set monitors. However, these exemptions allowed some vehicles to pass a Smog Check inspection even with a malfunctioning emissions control system. Not only does this undermine California’s clean air goals, but it also can result in costly, unexpected repairs for future owners of a vehicle with persistent unset readiness monitors.
Previous regulatory standards for unset monitors
Prior regulations allowed for the following readiness monitor exceptions:
| Vehicle fuel type | Model years | Readiness monitor exception |
|---|---|---|
| Gasoline | 1996-1999 | Any one unset monitor allowed |
| Gasoline | 2000 and newer | Only the evaporative monitor may be unset |
| Diesel | 1998-2006 | No unset monitors allowed |
| Diesel | 2007 and newer | Only the Diesel Particulate Filter (DPF) and the Non-Methane Hydrocarbon Catalyst (NMHC cat) may be unset |
Regulation changes
The new regulations require the completion of all monitors that can feasibly be set before a vehicle can pass inspection. However, the BAR’s implementation of these new standards will be gradual and data-driven:
The current pass/fail criteria (displayed in the chart above) will remain unchanged while BAR collects and analyzes Smog Check data.
Updates will only be made where data show unset readiness monitors can reasonably be expected to complete.
For some vehicles, requirements may never change.
Any updates will be phased in carefully, with input from the Smog Check industry and CARB.
The Smog Check Referee is available to assist motorists who encounter difficulties meeting new readiness monitor requirements.
Notification of any changes to the unset readiness monitor criteria will be announced well in advance of the start date for the changes.
More information
Industry stakeholders will be kept informed through BAR’s website, ET blasts, and other BAR communications.
Visit the On-Board Diagnostic Test Reference for a summary of pass/fail standards and instructions for inspecting vehicles with known test difficulties.
Review the Smog Check Reference Guide for program requirements as well as repair and retest procedures.
Learn more about the new readiness monitor regulations at Regulatory actions.