CARB: Onboard Diagnostic Program

Scope

This regulation applies to all heavy-duty vehicles operating in California.

Regulatory citations

• Title 13, California Code of Regulations, Sections 1965 to 1971

Key definitions

• California Air Resources Board (CARB): The state agency in California charged with controlling air pollution.
• Malfunction indicator lamp (MIL): A lamp on the dash that alerts the driver to a malfunction in the engine or emissions.
• Onboard diagnostic (OBD) system: A computer system (hardware and software) that monitors the vehicle’s engine and emissions system.
• Trouble codes (also known as diagnostic trouble codes, or DTCs): Computer codes stored within the OBD system that indicate the nature of a malfunction. There are temporary trouble codes (also known as pending trouble codes — the OBD has recognized a problem and is monitoring it) and permanent trouble codes (ones that require corrective action to reset).

Summary of requirements

The onboard diagnostic (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 California Air Resources Board (CARB) 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).