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U0100 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

The U0100 code represents lost signals between modules. It indicates that the electronic control module (ECM), also called the powertrain control module (PCM), is having communication issues with another particular module. 

Meaning

The U0100 code is a generic network communication code. It will cover all makes and models that are 1996 or newer. The PCM is the computer responsible for the engine’s management. The PCM communicates over a bus known as the controller area network (CAN). 

The CAN allows the PCM and other modules to communicate with one another. If the U0100 code appears, it means there are signals getting lost between the PCM and one or more of the other modules onboard the vehicle.

Causes

Issues with module communication is not a common problem. There are a few possible reasons why this code could appear. 

The code may appear due to a faulty PCM. It may also be due to problems with the control module circuit or the CAN bus. An installation of performance enhancing devices or chips that are not compatible with the PCM or CAN bus wiring also causes the lost communication code to appear.

If there is communication failure between anything within the network, you will get the U0100 code.  

Symptoms

A vehicle may experience a number of symptoms under the U0100 code. The check engine light will illuminate and the vehicle may stutter or stall. 

The vehicle may also completely shut down or not restart. After sitting idle for some time, it may start again. However, it would be risky to operate due to the possibility of another shut-down.

Other symptoms may be an increase in fuel consumption or a lack of power. 

Diagnosis

The U0100 code can show up periodically. It can also appear as a result from a dead battery. To start, clear the code and see if it comes up again. If the code returns, the next step is to carry out a visual inspection. 

Check for broken wires and loose connections. Also check for arcing, exposed PCM wires or any damage to wiring insulation. Upon the discovery of any issues, problems need to be repaired and the code cleared. 

If there is nothing discovered during inspection, check for technical service bulletins (TSBs). They are the repair and diagnostic procedure recommendations from the vehicle manufacturer. 

Common mistakes

Make sure to thoroughly diagnose other module communication related system codes. Misdiagnosis is known to occur when this problem code is diagnosed before others have been properly diagnosed and addressed. 

How serious is this?

The U0100 code is monitored when the vehicle’s ignition is on. The battery voltage needs to be at a certain level and other modules need to be correctly configured. The code indicates a serious situation. 

Vehicles with the U0100 code can experience sudden shutdowns while running. They may also not restart. Without the communication between modules, it will be difficult trying to drive the vehicle. 

What repairs can fix the code?

There are a number of things you can do to fix the U0100 code. First, make sure the battery is charged so the PCM has a power supply to function. 

There may be a problem with the CAN network if the vehicle has communication codes stored for multiple modules. If this is the case, the diagnosis will need to be shifted to check the integrity of the network. The CAN bus needs to be checked for circuit issues. A digital multimeter (DMM) can be used to check problems such as shorts to power and ground. 

You can also check the integrity of two terminating resistors located at both ends of the CAN bus. The resistors can be checked with a DMM set to ohms. When connected to the diagnostic port, the reading should be approximately 60 ohms if it is normal. You can check for shorts and opens in the same way.

Another action you can carry out is to use a breakout box to test the network. The breakout box tests the communication signals and listens to the network’s communication. It is connected to the diagnostic port. 

A diagnostic scan tool can also be used to try communicating with the PCM. It plugs into the diagnostic port and acts like another network module. If the PCM does not respond, check the circuit. A DMM can be used to check the integrity of the circuit and the PCM’s power and ground. If there is an open or short found, the problem needs to be isolated and then repaired.

Check the software before replacing the PCM. Sometimes, if you reprogram the PCM with software updates, it can work properly. If this does not work, then the PCM is likely faulty. You should replace and reprogram the PCM.  

Related codes

Codes U0100 to U0300 are related to lost communication with modules. Other “U” codes are related to network communication codes. 

Conclusion

The U0100 code indicates a serious issue. If the code is not cleared, the vehicle will have issues with starting up and can shut down.