Lost Communication with ECM/PCM “A”


If code U0100 appears, it means that signals are lost between PCM and one or more modules of the vehicle. 

Code U0100 refers to lost signals between blocks. This indicates that the Electronic Control Module (ECM), also known as the Powertrain Control Module (PCM), has communication problems with other specific modules. 


The U0100 code is a common network contact code. This includes all 1996 or newer products and models. The PCM is the computer responsible for the administration of the machine. The PCM communicates via a bus called the Control Area Network (CN). 

CN allows PCM and other modules to communicate with each other. If code U0100 appears, it means that signals are lost between PCM and one or more modules of the vehicle. 


signals are lost between PCM

Problems related to block communication are not a common problem. There are several reasons why this code appears. Code may appear due to incorrect PCM. This may be due to problems with the control module circuit or the CN bus. Installation of chips that are not compatible with performance-enhancing devices or PCM or even bus wiring will show lost communication code. 

If there is a failure of communication between anything on the network, you will receive the code U0100. 


Under code U0100 the vehicle can withstand many signals. The check engine lights flash and the vehicle may stumble or stop. The vehicle may stop completely or restart. After being idle for a while, it can start again. However, it is risky to take action due to the possibility of another closure. 

Other symptoms include increased fuel consumption or lack of power 


Code U0100 will be displayed from time to time. This will appear as a result of a dead battery. To get started, clear the code and see if it comes back. If the code comes back, the next step is to do a visual inspection. 

Check for broken wires and loose connections. Also check for any damage to the arcing, exposed PCM wire or wiring harness. Once any problems are detected, the problems should be fixed and the code cleared. 

If nothing is found during testing, check the Technical Service Bulletin (TSB). They are vehicle manufacturer repair and diagnostic procedure recommendations. 

Common mistakes 

Be sure to fully detect computer code related to other module interactions. This problem is diagnosed when the code is diagnosed and diagnosed before it is diagnosed. 

How serious is this? 

The U0100 code is monitored while the vehicle’s ignition is on. The battery voltage must be at a certain level and the other modules must be configured properly. The code indicates a serious situation. 

code indicates a serious situation

Vehicles with U0100 codes may experience a sudden shutdown while running. They cannot be resumed. Without contact between the blocks, it is difficult to try to drive. 

Can this repair code be fixed? 

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

There may be a problem with the network if the communication code is stored for too many volumes of the vehicle. If so, the diagnosis must be changed to check the integrity of the network. The CN bus should be checked for circuit problems. A digital multi-meter (DMM) can be used to investigate issues such as power and ground. 

digital multi-meter

You can also check the integrity of the two blocking resistors on either end of the CN bus. DMM set to ohms. With the resistor can be checked. When connected to a diagnostic port, it should be about 60 ohms if the reading is normal. You can check the shorts and open them the same way. 

The second action you can take is to use breakout to test the network. Breakout B checks communication signals and listens for network communications. It is connected to a diagnostic port. 

The diagnostic scan tool can also be used to try to communicate with the PCM. Here we use one of the best we use daily which is plugged into the search port and acts like another network module. If the PCM does not respond, check the circuit. DMM can be used to check the integrity of circuits and the power and ground of PCM. If it is open or narrow, the problem should be isolated and then corrected. 

Check the software before replacing the PCM. Sometimes, if you republish the PCM software with software updates, it will work properly. If this does not work, the PCM may be defective. You will need to repair and re-publish the PCM. 


Code U0100 indicates a serious problem. If the code is not cleared, there will be a problem starting the vehicle and they may stop. 

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