The California Air Resources Board (CARB) Onboard Diagnostic Program and associate regulations require the onboard diagnostic (OBD) system on a vehicle to meet specific performance standards, collect specific data, be fully functional, and not be tampered with. The U.S. Environmental Protection Agency (or U.S. EPA) has similar regulations at the federal level.

The OBD system on a vehicle continuously monitors the vehicle, engine, and emissions system, watching for malfunctions. When one occurs, a trouble code is created and stored in the OBD’s memory, and the driver is alerted via a malfunction indicator lamp (MIL).

Some trouble codes are temporary (also known as pending). In these situations, the OBD is monitoring the situation to see if the malfunction is real and if it continues. These codes can be cleared by a technician if the malfunction is no longer present.

Other codes are permanent codes, and these can only be corrected by addressing the issue that created the code.

Under the applicable CARB and federal regulations, tampering with the OBD to alter the engine performance in such a way that emissions are affected is not allowed. Also, any tampering that affects the emissions system or how trouble codes are collected, stored, and presented is not allowed.

The OBD system plays a key role in emissions testing in many states, including California. Under the Clean Truck Check program, a vehicle owner must download the OBD data from the vehicle and upload it to the CARB Clean Truck Check–Vehicle Inspection System (or CTC-VIS) database twice a year. Other states also use the OBD to verify emissions compliance, rather than doing the traditional smoke opacity testing (also referred to as the tailpipe smog check).