Diagnose Engine Code U0073: What It Means and What to Check

What U0073 Means

U0073 is a generic network code usually defined as "Control Module Communication Bus A Off." It means a module has detected that it can no longer communicate on one of the vehicle's data networks. Most modern vehicles use a Controller Area Network (CAN), a pair of wires that lets modules such as the engine computer, transmission controller, ABS unit and instrument cluster share information. The "U" prefix marks a network fault rather than a fault in a specific sensor or engine system.

The code is often described as an engine code because it appears during a scan or alongside a check engine light. It is really a communication fault and does not prove that any particular part has failed. Which physical network counts as "bus A," and which modules sit on it, varies by manufacturer, model and model year. Interpreting the code for a specific vehicle requires that vehicle's wiring diagrams and service information, which are not covered here.

Why a Single Code Can Have Many Causes

A communication bus is shared, so one fault anywhere on it can affect every module connected to it. A shorted wire, a corroded connector or a single failing module can pull the whole bus down. Other modules then report that they lost contact. For this reason, U0073 often appears together with other U-codes that name the modules that went silent, such as lost communication with the engine control module or the ABS module. The pattern of those codes is usually more useful than U0073 on its own.

Plausible Causes

Low or unstable system voltage from a weak battery, a charging-system fault, or loose or corroded battery terminals; Poor ground connections, which can disrupt module operation and network signals; Damaged, chafed or pinched network wiring, especially where harnesses pass near moving parts or sharp edges; Corroded or loose connectors, including water intrusion at modules or junction points; A failing control module that disrupts the bus for every other module; Aftermarket devices connected to the diagnostic port or spliced into the network, such as insurance trackers, remote starters, alarms or audio interfaces; Faulty termination resistors on networks that use them; Recent repair or collision work that left a connector unplugged or a harness damaged

What to Check Next

Start with the simplest and least invasive checks. Several common causes can be found without specialist equipment.

Check | What it involves | What the result suggests; Battery and charging system | Test battery condition and charging output; inspect and clean terminals and main cables | Low or fluctuating voltage can trigger network codes. Fix this first, clear the codes and recheck. Full-system scan | Use a scan tool that reads all modules, not only the engine controller; record every stored code | The modules that report lost communication, and the ones that do not respond at all, help locate the fault. Recent work or accessories | Consider what changed before the code appeared; disconnect aftermarket devices on the diagnostic port or network | If the fault clears with a device removed, the device or its installation is the likely cause. Visible wiring and connectors | Inspect harnesses and connectors at the modules involved for damage, corrosion or moisture | Physical damage or corrosion is a strong lead but should be confirmed with testing. Ground points | Check that main and module grounds are clean and tight | A poor ground can make a module drop off the network intermittently. Network resistance | With the battery disconnected, measure resistance across the two network wires at the diagnostic connector, using the vehicle's service specification | A reading far from the specified value suggests an open circuit, a short or a termination problem. Many high-speed CAN networks use two 120-ohm terminators, giving about 60 ohms, but confirm the correct value for the vehicle. Module isolation | Unplug modules on the affected bus one at a time while monitoring communication | If the network recovers when a module is disconnected, that module or its wiring is suspect.

Clearing the code and watching whether it returns gives useful information. A code that comes back immediately points to a persistent fault. One that appears only occasionally, such as in wet weather, over bumps or during cold starts, points to an intermittent connection or a voltage problem.

When to Stop Driving or Seek Professional Diagnosis

The seriousness of U0073 depends on which modules lose communication. On some vehicles the fault causes only a stored code or a few warning lights. On others it can affect systems that matter for safety. Stop driving and arrange inspection or towing if the code comes with any of the following:

Stalling, failure to restart, or serious loss of engine power; The transmission entering a reduced-function or limp mode, or shifting erratically; ABS, stability control, brake or airbag warning lights; Loss of power steering assist; An instrument cluster that goes blank or shows incorrect readings for speed or warnings

Professional diagnosis is advisable when basic checks do not find the cause. Confirming network signal quality often requires an oscilloscope, and locating a fault in a large harness requires model-specific wiring diagrams. Avoid replacing modules based on the code alone. A module is usually condemned only after its power, ground and network connections have been verified and the fault has been isolated to it. Some replacement modules also need programming or configuration before they will work.

Gaps to Keep in Mind

This guide describes the generic meaning of U0073 and general network diagnosis. It does not include manufacturer-specific definitions, technical service bulletins, known failure patterns for particular models, or exact test values for a given vehicle. Check those details in the service information for the specific make, model, model year and powertrain before carrying out repairs.