U0073 Trouble Code: Meaning & Diagnosis

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U0073 Definition: Control Module Communication Bus”A” Off

U0073 is a fairly common trouble code.  It is generic, so it has the same meaning for your vehicle as it would any other.  This code appears when something has interrupted the communication between two of your vehicle’s control modules.

The most common causes of U0073 are wiring problems (think open or short circuit).  It is rarely one of the control modules fails.  It can happen, though.

Symptoms

U0073 Symptoms

The symptoms associated with U0073 are going to depend on the vehicle manufacturer, as well as whether there is an intermittent communication problem, or if there is no communication at all between the control modules.

Common U0073 Symptoms Include:

  • Check Engine Light
  • Poor Fuel Mileage
  • Reduced Engine Power
  • Hard Start/No Start
  • ALL Dash Warning Lights On

Causes + Diagnosis

U0073 Diagnosis

In this section, you’ll find the most common causes of U0073.  They are presented in a way that anyone can intelligently diagnose this code.  

U0073 can be one of the more challenging codes to diagnose, so if you feel like you’re in over your head, don’t feel bad about contacting a professional.

1. Check for TSB’s Related to the Control Module

A great place to start diagnosing U0073 is to see if there are any Technical Service Bulletins related to the control modules.  

You can use this link to go to the NHTSA website VIN lookup tool.  From there you’ll be able to look up the TSB’s for your vehicle.  If you don’t have the VIN number handy, click the “vehicle” tab and you can search by year, model, and manufacturer.

2. Check for Other Codes

U0073 will often appear with other control module-related codes.  Clearing them will often clear U0073.  Make sure you evaluate those codes before tackling this one.

3. Inspect the BUS Communications Connections and Wiring

The first two steps are the due diligence that should be done to confirm that it’s not likely that another issue would be causing U0073.  Wiring issues are the number one cause of U0073.  Here’s how to go about finding what’s causing a wiring issue.

  1. Find the bus connections. 
  2. Inspect the wiring harness.  You are looking for chafing, burnt wires, or melted wiring/plastic.  Pay particular attention to anywhere that the harness needs to go around a bend or touches anything that could damage it.  You are looking for an open/short.  If there is damaged wiring, you’re going to need to repair the harness.  If there doesn’t seem to be any visible damage to the harness, you’ll need to move on to checking the harness connections at the control modules.
  3. Inspect the harness connections.  Once you’ve established that the wiring harness looks fine, the next place to look is where it plugs into the control module(s).  Unplug the harness from the control module.  Once unplugged, inspect the pins to see if there is any damage to them.  If one or more of them look discolored compared to the rest of the harness, that is likely to be what’s causing U0073.  You can use electric contact cleaner to properly clean the harness pins.  Check to see if the module can communicate with a scan tool.  If it can not the only thing left is one of the modules has likely gone bad.

Bad Control Module

It is rare that a bad control module will cause U0073, but it does happen.  We would recommend having someone taking a look at it before spending the money on a new one.

Conclusion

U0073 is usually caused by an issue with the wiring harness connection going in and out of the control module.