The bad: Any non-CARB-approved tune is considered to be a modification to a vehicle’s emission software and is, therefore, an illegal modification that will cause an emissions inspection failure.

The good: CARB-approved tunes are still legal. According to CARB's website, aftermarket manufacturers can obtain an Executive Order that shows its parts are still within CARB compliance. Specifically, CARB states, "Exempted parts are add-on or modified parts that have undergone a CARB engineering evaluation. If the part or modification is shown to not increase vehicle emissions, it is granted an exemption to emission control system anti-tampering laws. This exemption is called an Executive Order (EO) and allows the modification to be installed on specific emission-controlled vehicles. Every Executive Order part or modification has an assigned number that can be verified by Smog Check stations, BAR Referee stations, or by CARB."

According to CBAR, during a smog check, the vehicle is hooked up to a computer via its OBD-II port and data pulled via the protocol’s Mode $09. This mode, which has the friendly name “Request Vehicle Information,” pulls data like the VIN, ECU calibration info, and performance counters.

The test will then read the Calibration Verification Number (CVN) and Calibration Identification (Cal ID) stored in the vehicle’s ECU to determine if a software modification has occurred.

The CVN acts as a checksum, or thumbprint, of the ECU’s software. This is a computed number based on the parameters within ECU’s programming. When the parameters are changed via a software modification, the CVN will also change. A manufacturer is typically responsible for the method used to determine this checksum, however, simply loading a new software configuration will cause this checksum to change.

The Cal ID is akin to a software versioning number. Simply put, it’s often used by automakers to determine the software revision present on an ECU.