U0073 OBD-II: Control Module Communication Bus "A" Off

What does code U0073 mean?

Code U0073 stands for Control Module Communication Bus 'A' Off.

The computers in modern vehicles (referred to as modules) communicate with one another over a data bus. This bus is called the controller area network (CAN). CAN is a two-wire bus, consisting of CAN High and CAN low. CAN High has a communication rate, or baud rate, of 500k bits/second. CAN low on the other hand, has a communication rate of 125k bits/second. Information is transferred between the two buses by a gateway module.

The modules are connected in parallel with the CAN bus. Data is transmitted between modules sequentially, one bit at a time. This is referred to as serial communication. At each of the bus, there's terminating resistor.

When there’s a communication problem, the system will store a 'U' diagnostic trouble code (DTC). Code U0073 means one or more modules is not able to communicate properly over CAN bus 'A'. Depending on the manufacturer, bus 'A' can refer to either the CAN High or CAN Low bus.

U0073 symptoms

• An illuminated check engine light
• A secondary code specifying a failed module
• Performance problems ranging from a no-start condition to inoperative HVAC system, depending on which module(s) are unable to communicate

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Common causes for U0073

Code U0073 is typically caused by one of the following:

• A faulty control module
• A problem with the CAN bus

How to diagnose and repair U0073

Perform a preliminary inspection

Sometimes U0073 can pop up intermittently, or it can result from a dead battery. Clear the code and see if it returns. If it does, the next step is to perform a visual inspection. A trained eye can check for issues such as broken wires and loose connections. If a problem is found, the issue should be repaired and the code cleared. If nothing is discovered, check for technical service bulletins (TSBs). TSBs are recommended diagnostic and repair procedures put out by the vehicle manufacturer. Finding a related TSB can greatly reduce diagnostic time.

Check for a faulty control module

After the preliminary inspection, an expert will check for a defective control module. The first step in doing this is to check for any other module specific DTCs stored. For example, there may also be a code U0100 stored, indicating a communication problem with the Powertrain Control Module (PCM).

Next, a roll call of the modules can be performed using a diagnostic scan tool. The scan tool is connected to the vehicle through the onboard diagnostic port. Once it's connected to the vehicle, the scan tool becomes another module on the network, communicating back and forth. The scan tool can then perform a roll call of the other modules, requesting their status to see which ones are working properly.

Any module that doesn't respond is likely defective, or has a problem with its circuity. A non-communicating module doesn't necessarily mean the module is bad. The module may not have a complete circuit with both power and ground. Or, it may need to be reprogrammed.

Next, an expert will go through and unplug the modules one at a time. If unplugging a certain module restores network communication, there's a problem with the module or its circuitry.

Check the network

Initial inspection of network integrity is done at the diagnostic port with a digital multimeter (DMM). Connecting the meter to the CAN High and CAN Low pins on the diagnostic port can reveal opens, shorts and high resistance in the CAN bus.

A DMM at the diagnostic port can also be used to check the two CAN bus terminating resistors. Typically, if one of these resistors fails, the bus will still work. But if both fail, the bus will shut down. A DMM reading of 60 ohms indicates the resistors are intact.

If available, a breakout box can also be used to test the network. This tool connects directly to the vehicle's diagnostic port. A breakout box is a measurement tool used to listen to bus communication and test CAN network signals. Among other things, it can be used to pinpoint a faulty module or inoperative branch of the CAN network.

Other diagnostic codes related to U0073

• U0074: Code U0074 means one or more modules is not able to communicate properly over CAN bus 'B'

Code U0073 technical details

U0073 is monitored when the supply voltage to the modules is in the normal operating range and the vehicle is running.