U00xx OBD-II Codes (U0000–U0099)

High Speed CAN Communication Bus

U0073 is a generic SAE-defined network (U-series) diagnostic trouble code that means "Control Module Communication Bus 'A' Off." Modern vehicles let their control modules (PCM/ECM, TCM, ABS, BCM, and others) talk to each other over a serial data network, typically a Controller Area Network (CAN) bus. U0073 sets when a module determines that communication on bus "A" has stopped — either the bus has gone "off" (no valid traffic) or the module can no longer transmit/receive on it. Because the fault is on the network itself rather than in one sensor, U0073 is frequently accompanied by several "lost communication" codes (for example U0100 ECM/PCM, U0101 TCM, U0121 ABS) as each module reports that it can no longer hear the others. The "A" designates a specific bus circuit; the related code U0074 covers bus "B." While the U0073 label and bus-off meaning are standardized, exactly which physical wires and modules sit on "bus A," and the precise enabling conditions, vary by manufacturer, so the make-specific service information and wiring diagram should be confirmed during diagnosis. The codes immediately above this range, U0075 through U00FF, are ISO/SAE reserved.