OBD-II Code U0073 — Control Module Communication Bus Off

What Does This Error Mean?

U0073 means the CAN bus — your car's internal communication network — has completely shut down. It's not just one module missing; the whole communication line has gone offline. This is more serious than most U-codes because every module on that bus is affected at once. Your car may not start, multiple warning lights will illuminate, and many systems will stop working together. This code requires immediate attention.

Affected Models

All CAN bus vehicles (2004+)
Common in GM trucks and SUVs
Common in Ford vehicles
Common in Dodge/Chrysler products
Common in VW, Audi, and BMW

Common Causes

Short circuit on the CAN bus wiring — the network shuts down to protect itself from damage
One module with an internal failure causing it to jam the communication line
Severely corroded or damaged main wiring harness causing a bus short
Aftermarket device wired directly into the CAN bus wiring and creating a fault
Water intrusion shorting out a major connector or junction block on the CAN network

How to Fix It

Disconnect the battery for 20-30 minutes and reconnect. Some temporary bus-off conditions caused by voltage spikes or software glitches will clear with a full reset. Clear all codes and take a 20-minute drive before concluding it's a hard fault.

If the code returns within a drive cycle, you have a real electrical problem that needs tracing.
Inspect the OBD-II port under the dashboard. Pins 6 and 14 are the CAN High and CAN Low lines. Look for any debris, bent pins, or signs of a device that was forcefully inserted. An aftermarket device stuck in the port can kill the bus.

Remove any dongles, insurance trackers, or code readers from the OBD-II port before diagnosing U0073.
With the ignition off and battery disconnected, measure resistance between pins 6 and 14 of the OBD-II port. You should read approximately 60 ohms. Much lower means a short; much higher means an open circuit. Either is a problem.

A basic multimeter is all you need for this test. They're available at any hardware store for under $30.
Inspect the main engine wiring harness and look for any obvious damage — melted sections, areas rubbed raw against metal, or connectors that have gotten wet. Pay attention to harness sections near the exhaust, which can cause insulation to melt.

Check especially near the firewall grommet where the harness passes from the engine bay into the cabin.
If you can't find obvious wiring damage, a shop will need to do a module isolation test — disconnecting modules one at a time until the bus comes back online. The module that was last disconnected when the bus recovers is the culprit.

This test can take 1-3 hours of labor time. It's methodical but necessary to find an internal module fault.

When to Call a Professional

U0073 is one of the harder codes to diagnose without professional equipment. It requires measuring resistance on the CAN bus and methodically disconnecting modules one at a time to find which one is pulling the bus down. This is time-consuming work — budget 2-4 hours of labor for diagnosis alone. A shop with an oscilloscope can see the bus activity in real time, which speeds up diagnosis significantly.

Frequently Asked Questions

What does 'bus off' actually mean?

The CAN protocol has a built-in safety feature: if errors accumulate past a certain threshold, the bus shuts itself down completely — this is called 'bus off.' It's a protection mechanism to prevent one failed device from corrupting data for all others. Finding and fixing the device that caused all those errors is the key to resolving U0073.

Can a bad ground cause U0073?

Yes, absolutely. The CAN bus relies on stable reference voltages. A bad chassis ground can inject electrical noise into the network, causing errors to pile up until the bus shuts down. Always check battery grounds and chassis ground straps before doing more complex diagnosis.

My car won't start and has U0073. What now?

U0073 with a no-start condition is a red flag — the ECM or BCM may not have enough network communication to authorize the ignition. Have the car towed to a shop rather than attempting roadside fixes. This is not a code you can easily resolve without the right tools.