Severity: High Code type: Generic (all makes)

U0073 Code: Control Module Communication Bus "A" Off

A module saw so many errors on CAN bus A that it shut off its own transmitter.

Stop driving

If the car won't start, stalls, or the transmission, ABS and power steering warnings are all on, don't drive it. If U0073 is stored as history and everything works normally, you can drive to a shop, but expect the fault to return when the damaged wire moves or gets wet.

At a glance

Severity: High

Meaning: Control Module Communication Bus "A" Off
System: Module Communication · Network
Safe to drive?: With caution
Typical repair cost: $75–$1,500
DIY difficulty: Hard

Every module on a CAN network counts transmission errors. When a module's error counter climbs past a fixed limit, it deliberately goes into a 'bus off' state and stops talking so it can't jam the network, then logs U0073. Unlike a single lost-communication code, U0073 points at the bus itself: a short between CAN-high and CAN-low, a line shorted to ground or battery voltage, a missing terminating resistor, or one faulty module dragging the whole network down. Because many modules share the same two wires, one fault can make a car behave as if half its electronics failed at once.

How to read U0073

U Network
0 Generic (SAE)
0 Module Communication
73 Specific fault

Symptoms

Many warning lights on at once (check engine, ABS, airbag, traction, charging)
Instrument cluster goes dark, resets or gauges sweep randomly
No-start or no-crank condition
Transmission in limp mode, power steering assist lost on electric systems
Scan tool shows several modules as 'no response'
Symptoms come and go with rain, car washes, bumps or steering movement

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

CAN wiring shorted to ground, to battery voltage or to each other (chafed harness, crushed wire, bad repair) 32%
Water or corrosion in a connector or splice pack on the bus 22%
One failed module pulling the bus down 18%
Aftermarket electronics spliced into the CAN lines (remote start, alarm, radio interface) 12%
Open or failed terminating resistor (often inside a module) 8%
Very low system voltage during cranking or charging faults 8%

Diagnosis step by step

Map which modules respond
Run a full network scan and write down which modules answer and which don't. Compare against the wiring diagram: if every module on one branch is silent, the fault is likely on that branch or at the splice that feeds it.
Tools: OBD-II scanner
Check resistance between the CAN lines
With the battery disconnected, measure between OBD pins 6 and 14. About 60 ohms is normal; roughly 120 ohms means one terminator or one end of the bus is open; a very low reading means CAN-high and CAN-low are shorted together.
Tools: Multimeter
Check each line for shorts to ground and power
Still with the battery disconnected, measure from pin 6 and from pin 14 to chassis ground; you should see high resistance (thousands of ohms), not a few ohms. With the key on, each line should sit roughly around 2.5 V; a line stuck at 0 V or near 12 V is shorted.
Tools: Multimeter
Unplug modules one at a time
With the key on and a meter or scan tool connected, disconnect modules on the affected branch one by one. When the bus recovers after a particular module is unplugged, that module or its pigtail is the culprit.
Tools: Multimeter, OBD-II scanner, Screwdriver
Inspect harness routing and splice points
Look for rubbed-through insulation along seat tracks, under carpets, at door hinges, near the steering column and anywhere the harness passes sheet metal. Check splice packs and connectors in areas that get wet, like under the battery tray or behind the kick panels.
Tools: Flashlight
Remove aftermarket connections
Remote starters, alarms and radio adapters are often spliced directly into CAN wires. Remove them and restore the factory wiring, then clear codes and retest.
Tools: Pliers, OBD-II scanner

Where the parts are

Charging system and control modules

1Battery
2Alternator
3Engine computer (ECM)
4CAN bus
5Transmission
6Wheel speed sensor
7High-voltage battery

Repair options & costs

Repair	DIY?	Estimated cost
Network diagnosis (shop labor)	No	$150–$400
Repair chafed or shorted CAN wiring	No	$150–$700
Clean or replace corroded connector/splice	No	$100–$400
Remove or reinstall aftermarket device correctly	Yes	$75–$300
Replace faulty module (incl. programming)	No	$400–$1,500

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Field data

Chevrolet		16	Equinox, Cobalt, Bolt
Toyota		7	Camry Hybrid, Camry, Prius Plug-In

Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.

Related codes

Often appears together with

U0100 Network Another module stopped hearing from the engine computer on the CAN network. High U0121 Network Other modules stopped receiving data from the ABS module, so ABS and stability control are off. Moderate U0101 Network The engine computer and other modules stopped hearing from the transmission control module. High U0155 Network Modules can't reach the instrument cluster, so gauges, warning lights and chimes may go dead. Moderate P0562 Powertrain The engine computer sees too little supply voltage – usually a charging, battery or ground… Moderate C0131 Chassis The GM brake module sees an implausible or out-of-range signal from the brake pressure sensor. Moderate U0001 Network A module found the high-speed CAN network itself misbehaving, not just one silent module. High

Check engine light on Car won't start Limp mode ABS / traction light on Harsh or slipping shifts Network

Emissions / smog test impact

Depends

Whether U0073 fails a US emissions test depends on whether it turns on the check engine light and whether the test equipment can still talk to the PCM. If the network is down enough that the station can't read readiness data, the vehicle is typically rejected.

Frequently asked questions

What does 'bus off' actually mean?

It's a self-protection mode built into every CAN module. After too many failed transmissions, the module stops sending so it doesn't flood the network, and records U0073. It's a symptom of a noisy or shorted bus, not proof that the module reporting it is bad.

Can one bad module cause U0073 in the whole car?

Yes. A module with an internal short on its CAN transceiver can hold the lines at the wrong voltage and knock every other module off the network. Unplugging modules one by one is the standard way to find it.

Why does U0073 show up after rain or a car wash?

Water getting into a connector or splice pack can bridge the CAN wires or short them to ground. The fault then disappears as it dries, which is why checking wet-area connectors and seals is a key step.

How is U0073 different from U0100?

U0100 says one specific module (the engine computer) went silent. U0073 says the bus itself became unusable for the module that logged it, so the search starts with the wiring and all connected modules rather than one computer.

Sources

SAE J2012 – Diagnostic Trouble Code Definitions
ISO 15031-6 – Road vehicles, diagnostic trouble code definitions (EOBD)
Manufacturer public service information (cross-checked)
NHTSA Complaints API
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