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U0073 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.

Definition

U0073 is an OBD-II Code that refers to Control Module Communication Bus "A" Off

Meaning

A U0073 code is caused by the Control Module Communication Bus "A" Circuit having communications trouble. 

A data bus is used by modern automobiles' computers (also known as modules) to communicate with one another. The CAN bus modules are linked in parallel. 

Data is transmitted from one module to the next bit by bit. Serial communication is the process of sending data from one module to the next. There's a terminating resistor at each of the bus terminations.

When there's a problem with communication, the system will record a 'U' diagnostic trouble code (DTC). The code U0073 indicates that one or more modules is unable to communicate efficiently over CAN bus 'A.' The term "bus A" can refer to either the CAN High or CAN Low bus, depending on the manufacturer.

Causes

Common causes for this code include:

Depending on which module(s) are unable to communicate, a variety of issues can arise, ranging from a no-start scenario to a broken HVAC system.

A check engine light that is illuminated

A secondary code identifying a failed module

Symptoms

The symptoms of a U0073 code are:

  • A Check Engine light that's illuminated
  • An engine that won't start
  • A vehicle that shuts off while driving
  • An HVAC system not working properly
  • On the instrument cluster display, all of the indicator lights on
  • Poor fuel efficiency

Diagnosis

To diagnose a U0073 DTC code, a technician would:

1. Make sure the battery has a full charge, and that connections are clean and tight.

2. Using a scan tool, read any DTCs stored in the system memory and compare it to manufacturer-specific diagnostics code(s).

3. Confirm with another scan tool if there's continuity at all of the modules on CAN bus 'A.' 

4. Verify every module is able to communicate properly with another diagnostic computer while driving (if possible).

5. Troubleshoot each component until one of them fails, or substitute similar components from another vehicle (ECM) that's configured identically (same VIN number).

6. If an ECM is replaced, clear all codes and test drive the vehicle.

Common mistakes

The following are common mistakes when diagnosing the trouble code U0073:

  • Basing a diagnosis on one diagnostic computer.
  • Trying to troubleshoot a system that's disabled or not working properly due to another issue.
  • Using an old DTC from the vehicle's history as a reference point.
  • Replacing parts without confirming compatibility with module(s) and/or manufacturer specifications first. 
  • Unplugging and replugging connectors, if they were loose in the first place.
  • Coding out incorrect components due to poor communication at CAN bus 'A.' 
  • Taking too long to reach a conclusion. 
  • Not hooking up analog gauges for testing during diagnostics (if possible). 
  • Bypassing important steps along the way, such as checking voltage levels, resistance, continuity, etc.
  • Not checking for problems with the transmission, engine speed sensor, or O2 sensors while driving.
  • Replacing parts randomly instead of following a specific plan.

How serious is this?

A U0073 code is serious. It's a major OBD-II trouble code and usually requires an extensive diagnosis to determine the cause. One or more modules will need to be replaced and/or recorded.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Replacing a failed ECM / PCM module / control unit. 
  • Replacing a bad CAN High or Low bus wiring harness. 
  • Re-flashing the latest calibration to the vehicle's modules through reprogramming, if necessary. 
  • Finally, in some cases, replacing faulty sensors and/or complete powertrain components are required for the system to communicate properly again.

Related codes

A U0073 is related to and may be accompanied by the following codes:

  • U0047
  • U0100
  • U0101
  • U0102  (Also known as U0201, U0202, and U0203)

Most of these codes may be accompanied by a second digit to identify the exact module. For example: U1050 (throttle position sensor), U2102 (RPM signal circuit range/performance), or A112 (VSS sensor).

How much does it cost to fix the U0073 code?

The repair cost of a U0073 code varies, depending on the make and model of the vehicle. Costs may range from around $100 to more than $1,000. In uncommon cases, two or more modules may need to be replaced.

Conclusion

In conclusion, the U0073 code is an OBD-II trouble code. It's a major DTC that requires extensive diagnostics to identify the underlying cause. One or more modules will need to be replaced and/or recorded. Luckily, there are solutions that may fix this problem completely.

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