U0073 Code: CAN Bus A Off — Causes and Diagnosis

Quick Answer

U0073 means a control module detected that communication bus “A” entered a bus-off state. The fault can affect several modules at once, so the module that stored U0073 is not automatically the failed module. Begin with battery and charging-system condition, then save a full-system scan and identify which modules are offline. Inspect shared power, grounds, connectors, and the exact CAN network shown in the OEM wiring diagram before replacing any controller.

Code definition	Control Module Communication Bus “A” Off
What “bus off” means	A controller has stopped normal transmission after the network recorded excessive communication errors. OEM diagnostic logic and network names vary.
Typical severity	Moderate to high. The vehicle may operate normally, enter reduced-power mode, fail to start, or lose functions such as gauges, ABS, stability control, transmission control, or power steering assistance.
Best first evidence	Battery voltage and state of charge, all-module scan report, code status, modules that will not communicate, and OEM network topology

What U0073 Means

Modern vehicles use controller area networks to exchange engine speed, wheel speed, torque requests, gear position, brake commands, and other data. A high-speed CAN network normally uses a twisted pair of wires and termination designed for that vehicle's topology. A short, open, power or ground failure, or a module transmitting invalid electrical signals can produce enough errors for a controller to remove itself from the bus. That protective state is called bus off.

“Bus A” is a generic label, not a universal wire color or connector pin. One manufacturer may map U0073 to a high-speed powertrain network, while another may call the same network F-CAN, HS-CAN, CAN-C, or CAN Bus 2. Some vehicles route communication through a central gateway, which means a scan tool can reach one side of the gateway while modules on another branch remain silent.

U0073 is not a throttle-actuator code. It may appear beside throttle, transmission, ABS, or engine codes because those systems share data, but the network fault has to be diagnosed on its own evidence.

Symptoms and Warning Patterns

• Several warning lights or messages appear at the same time.
• The gauges drop out, sweep, freeze, or show implausible values.
• The engine will not crank, starts and stalls, or enters reduced-power mode.
• The transmission shifts harshly or stays in a default gear.
• ABS, stability control, adaptive safety features, or electric power steering become unavailable.
• A scan tool cannot connect to one or more control modules.
• The fault is intermittent and follows vibration, rain, a turn, engine movement, or recent accessory installation.

A stored history code after a weak-battery event needs a different approach from an active code with several modules offline. Record whether U0073 is current, pending, history, or intermittent before clearing it.

Common Causes

1. Low system voltage during cranking, a discharged battery, or unstable charging voltage causes modules to reset at different times.
2. A shared fuse, relay, power feed, splice, or ground connection prevents one or more modules from waking and communicating.
3. CAN high or CAN low is shorted to ground, shorted to voltage, shorted to the other network wire, or open at a connector or splice.
4. Harness insulation has chafed against a bracket, A/C line, body seam, or engine component.
5. Water, terminal fretting, backed-out pins, or poor connector tension creates an intermittent connection.
6. An aftermarket alarm, remote starter, audio interface, tracker, trailer controller, or diagnostic dongle is connected incorrectly or loads the network.
7. A gateway or control module has lost power or ground, contains incorrect software, or has an internal electrical failure. Module failure is possible, but it should be considered after circuit tests.

Diagnose U0073 in This Order

1. Confirm battery state of charge and charging-system operation. If P0562, U3003, or other low-voltage codes are present, repair the voltage problem first. A network diagnosis performed with unstable supply voltage can create misleading results.
2. Run a complete system scan and save the report. Note which module stored U0073, which modules do not respond, and every related U-code. A group of missing modules on one branch often narrows the fault more effectively than U0073 alone.
3. Check the VIN, OEM bulletins, wiring diagrams, connector views, network topology, and ground locations. Determine what the manufacturer calls bus A on this vehicle and which gateway, splice packs, and terminating modules belong to it.
4. Inspect recent work and aftermarket equipment. Remove unsupported devices from the diagnostic connector, and verify any splices made for alarms, trackers, audio equipment, or remote starters. Do not cut factory network wires to “test” the circuit.
5. Inspect fuses, module power feeds, grounds, and connectors for the offline branch. Voltage-drop testing under load is more useful than a visual check of a ground eyelet. Wiggle testing can help reproduce an intermittent fault when the scan report or scope remains connected.
6. With the vehicle powered down according to the service manual, test network resistance and shorts at the specified connector. The familiar 60-ohm check applies only to networks with two 120-ohm terminators visible from the chosen test point. Gateways, isolated branches, wake-up circuits, and CAN FD networks can produce other valid readings, so use the OEM value.
7. If static checks pass, view CAN high and CAN low with a suitable oscilloscope. Look for missing bias, clipped or mirrored waveforms, noise, or a branch that collapses when a component wakes. Isolate branches or modules only in the order authorized by the manufacturer to avoid new faults and connector damage.
8. Repair wiring with the OEM splice method, restore harness routing and retainers, then let the vehicle complete any required sleep and wake cycle. Clear codes, rescan every module, and verify the repair during the conditions that originally triggered the failure.

Brand and OEM Differences

OEM example	Diagnostic lesson
General Motors	GM bulletin 23-NA-102 lists U0073 as “CAN Bus 2 Off” on covered diesel trucks and SUVs. The documented causes include harness contact at an alternator bracket, water at a connector, and loose grounds. The same physical damage can set many lost-communication and invalid-data codes, so the whole code cluster matters.
Honda	A Honda VSA service article describes U0073 or U0028 with other codes as an F-CAN bus-off fault. Its listed causes include CAN-high shorts, a bad F-CAN control unit, and poor power or ground to a related controller. Honda's F-CAN name should not be carried over to another make.
Hino	Hino's heavy-duty OBD specification shows its U0073 monitor enabled with battery voltage between 11 and 16 volts and a sustained bus-off condition. Those thresholds describe that application, not a universal test specification.

Repair Cost Factors

Cost depends primarily on diagnostic time, harness access, connector availability and whether programming is required. Get a written estimate after the failed circuit or module is identified; U0073 alone cannot support a parts quote.

U0073 FAQ

Related Information

Low supply voltage often appears beside communication codes. See the P0562 system-voltage diagnosis guide and the OBD-II code basics guide.