U0073 Code: Control Module Communication Bus A Off

What the U0073 code means

Every module on a CAN network counts the errors it detects while sending and receiving messages. If the counter goes past a limit, the module enters the bus off state: it stops transmitting so it does not interfere with the others. When it logs that event, it stores the U0073 code.

The letter A identifies the network inside the vehicle. Each manufacturer assigns the letters to its own networks, for example a high speed network for the powertrain and another one for chassis or comfort. The wiring diagram tells you which modules hang off each one.

The causes are the same as for any network fault: CAN wires shorted or open, a defective module that sends garbage, moisture in a connector, an accessory plugged into the network or unstable battery voltage. If the fault is intermittent, it tends to show up in the rain, over bumps or at startup.

Work the diagnosis in this order: 1) battery and alternator, because low system voltage is the number one cause of bursts of U codes; 2) the module list on the scan tool, to see which module is not responding; 3) power, fuses and grounds of the module that is not responding; 4) network resistance at the OBD2 connector, about 60 ohms between pins 6 and 14 with the battery disconnected; 5) connectors and wiring at the module, especially in areas exposed to moisture such as under the carpets, in the doors and in the trunk.

Measuring the network and isolating the module that is bringing it down is the heart of any U code diagnosis, so take the time to do those steps in order before you replace anything.

U0073 code symptoms

• Many warning lights on at the same time.
• Several U codes stored in different modules.
• Functions that stop responding on and off.
• The engine may not start or may go into limp mode if the powertrain network is affected.

Likely causes of the U0073 code

1. Network wiring shorted or open: chafed, pinched or soaked with moisture.
2. A module with an internal fault that drags the network down: you find it by unplugging modules one at a time.
3. An accessory plugged into the network: alarms, trackers or OBD2 adapters left plugged in permanently.
4. Weak battery or unstable voltage: it causes errors during startup.
5. Damaged termination resistor: the network resistance drifts away from 60 ohms.

How to diagnose the U0073 code step by step

1. Write down which modules store the code and which ones do not respond to the scan tool.
2. Measure the network resistance. You should see about 60 ohms between OBD2 pins 6 and 14 with the battery disconnected, if that network reaches the connector.
3. Measure the voltage on CAN high and CAN low with the key on. Each should read close to 2.5 V.
4. Unplug the modules one at a time until the network recovers.
5. Inspect the wiring in areas where it rubs or gets wet and remove any added accessories.

How to fix the U0073 code

• Repair the wiring and keep the twist in the wires.
• Replace the module that is bringing the network down.
• Remove poorly installed accessories.
• Clear the codes in all modules and verify with a road test.

Technical reference data

• Network resistance: about 60 ohms (two 120 ohm terminations in parallel).
• CAN high: 2.5 to 3.5 V. CAN low: 1.5 to 2.5 V.
• These values are a guide only, so check the manufacturer data for your vehicle.

Brand-specific notes

After you fix the U0073 code, clear the codes in all modules. Many of them keep old U codes from the time the fault was present. Accessories installed after the sale, such as alarms, GPS trackers and OBD2 adapters left plugged in permanently, are a frequent cause of network problems.

Frequently asked questions about U0073