Common U OBD2 Trouble Codes & How to Fix Them – Part 1

Technical

Nov 21st, 2021

Common U OBD2 Trouble Codes & How to Fix Them – Part 1

OBD2 Diagnostic Trouble Codes can be pretty informative, but only if you know what they mean. In this guide, we’ll go through some of the most common OBD 2 trouble codes that start with ‘U’, signifying a problem with the car’s network system.

Let’s dive right in.

About User Network (U) OBD2 Diagnostic Trouble Codes

If you’re reading this article, there’s a good chance that you already know a bit of OBD2 trouble codes. Still, you can brush up on that knowledge by checking out our in-depth introduction to OBD2 here

In a nutshell, OBD2 error codes are your car’s way of telling you that it has a problem. Even though the codes aren’t perfect, they’ll give you an excellent sense of the problem and which exact part of your car is affected.

As you might know, each OBD2 diagnostic trouble code (DTC) consists of five characters, with the first alphabet representing:

  • P for Powertrain
  • B for Body
  • C for Chassis
  • U for ‘Undefined’ or User Network/Network Communication

What Are User Network OBD2 Error Codes?

DTCs beginning with U are known by several names, to wit, Undefined, User Network, or Network Communication. These codes refer to problems related to shared functions among the car's computer and other systems.

Another way of looking at it is that these miscellaneous codes don’t fall under the other categories (P, B, or C).

Now that you're up to speed, let's take a quick look at some of the most common U OBD2 codes, and what you can do to fix them.

Common User Network OBD2 Error Codes & How to Fix Them

OBD2 DTC U0101

Affected part: Transmission Control Module (TCM)

What is likely happening: What’s likely happening with this code is that the Transmission Control Module (TCM) is no longer communicating with other modules in your car.

The problem has several possible causes, including open electrical circuits or those that have shorted out. Although less likely, it’s also possible that the control module itself has failed.

Symptoms: When this code is active, you might see the Malfunction Indicator Lamp turn on while the car fails to shift gears. Besides that, the car might be stuck in one particular gear, typically the 2nd or 3rd gear.

How to fix the problem: The solution is to inspect the Transmission Control Module (TCM) to ensure it's working. Then, you should also check the surrounding electrical parts, like wiring and fuses, for any damage or faults.

OBD2 DTC U0200

Affected part: Door Control Module B

What is likely happening: The most likely reason behind this code is that the Door Control Module B has lost its connection with other control modules in your car.

Symptoms: Windows stuck and can’t move are some of the common signs accompanying this code. Besides that, the door's lock might also fail to engage or disengage.

How to fix the problem: The most common cause is a fault in the electrical circuit, so that’s where your attention should be. Check the wiring for any breaks or other signs of damage, then ensure that the circuit is indeed receiving the power it needs to function correctly.

OBD2 DTC U0120

Affected part: Starter Generator Control Module

What is likely happening: The most likely reason for this code to appear is that the Starter Generator Control Module is not communicating with other modules through the car’s network.

Symptoms: You can look out for several accompanying signs when this error code is active. The Malfunction Indicator Lamp would likely be turned on for starters, and the vehicle will struggle to start or stay running.

If you’re driving a hybrid, there are some unique symptoms to look out for. For instance, the Hybrid Warning Indicator would turn on, and the car would only run using its gasoline engine.

How to fix the problem: When resolving the issue on a hybrid vehicle, pay extra caution. The network system in hybrid vehicles is a high voltage system, so it's much riskier than working on a conventional car.

As you troubleshoot this issue, you’ll need to check all the electrical connections and wiring attached to the Starter Generator Control Module. Those connections should be solid and free from any sediment buildup, and the wires should also be free from any damage. Plus, check the fuses if you need to replace any of them.

OBD2 DTC U0231

Affected part: Rain Sensing Module

What is likely happening: The cause behind this diagnostic trouble code is that the Rain Sensing Module of your vehicle is not communicating with the other control modules in your car’s network.

Naturally, this code only affects vehicles that have a Rain Sensing Module as part of their systems.

Symptoms: When the Rain Sensing Module is affected, the car’s wipers will become problematic. For instance, the wipes won’t turn on or off, and they do not react automatically to raindrops on the windscreen as they’re supposed to.

How to fix the problem: To fix this problem, you must determine if the Rain Sensing Module has shorted or grounded out, as those are the most likely electrical faults triggering this trouble code.

Once you’ve ruled that out, check the wiring and fuses to ensure that none of them is damaged and fail to allow power to flow to the module correctly.


To learn more about OBD 2 trouble codes and the most common ones you’re likely to experience, check out the blog at carpart.com.au. There, you’ll discover plenty of updates surrounding Diagnostic Trouble Codes and other issues affecting drivers like you in Australia and beyond!


By Ray Hasbollah