OBD-2 vs. SAE J1939: What’s the Difference Between These Vehicle Diagnostic Protocols?

OBD-2 is the diagnostic standard used in passenger cars and light trucks. SAE J1939 is the diagnostic standard used in heavy-duty trucks, buses, and off-road equipment. The main difference is how they communicate: OBD-2 answers questions when asked, while J1939 continuously broadcasts data to the whole vehicle network.