OBD-II Trouble Codes

U0100 Code: Lost Communication with ECM/PCM “A”

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Most people nowadays understand computer networks on some level even if they don’t understand the nuances of network communication. One computer communicates with another within an office environment or even within households by sharing, displaying, and storing information.

Onboard vehicle computer networks are the order of things these days and began appearing on high-end vehicles in the early 1980s. Late model vehicles have a growing number of onboard computers (also known as “nodes” or “modules”) that share information among themselves. To do so, the various computers are connected to a vehicle network called a “bus.” Many vehicles have more than one network bus, and the different networks on a vehicle communicate at different speeds because they serve different purposes. 

These multi-network vehicles typically have a “Gateway” module that enables the different networks to communicate with each other. This is necessary because each network has a slightly different “protocol” or language and decisions must be made concerning the priority each module and message has on the network.

Interior of a modern automobile
Late model vehicles have a growing number of onboard computers (also known as “nodes” or “modules”) that share information among themselves.

Computers on a vehicle’s high-speed network handle time-sensitive functions where data sharing and decision must be done very rapidly. Examples would be engine control and vehicle dynamics functions like Electric Power Steering, Anti-Lock Brakes and Traction Control, Vehicle Stability, and Advanced Driver Assist. On some vehicles, the anti-lock braking system (ABS) may share wheel speed information with the powertrain control module (PCM), which then shares it with the instrument cluster. This theoretically reduces the number of sensors and the amount of wiring.

In the realm of diagnostic trouble codes (DTCs), there are several categories. Pxxx codes are related to the powertrain (engine and transmission). Bxxx codes are body computer-related codes, while Cxxx codes are chassis-related codes. And finally, Uxxx codes are a warning that there is some kind of network communication failure between modules.

In other words, a Uxxx code is stored when one or more modules aren’t communicating with the others. This may appear as an inoperative instrument cluster (for a prime example) or there may not be any noticeable symptom at all.

Your vehicle’s communication data bus serves as the line of communication between different modules in your vehicle, including its primary computer or PCM. Without this line of communication, the different modules in your vehicle can’t send signals to one another. U0100 is among the generic trouble codes related to network communication issues.

engine control module 3
Your vehicle’s communication data bus serves as the line of communication between different modules in your vehicle, including its primary computer or PCM.

What Does the U0100 Code Mean?

DTC U0100 stands for “Lost Communication with ECM/PCM ‘A.’” It is triggered once there is a loss of communication between your PCM and a particular module. Once this happens, the vehicle may shut off and it may not restart unless you address the issue triggering the code. 

Note that the exact definition of trouble code U0100 may vary based on the vehicle manufacturer.

CAN Bus 

Your PCM communicates to other modules in your vehicle via a data bus. On many vehicles, the modules use a protocol called controller area network (CAN) to talk to one another. So, the bus is known as the CAN bus.

Basically, it works just like your body’s nervous system. The CAN bus system is wired throughout your vehicle. Without it, the different modules in your vehicle, such as the PCM, ABS, transmission control module (TCM), or body control module (BCM), won’t be able to communicate with one another. 

abs control module of brake system
Your PCM communicates to other modules in your vehicle via a data bus. On many vehicles, the modules use a protocol called controller area network (CAN) to talk to one another.

What are the Possible Causes of the U0100 Code?

Code U0100 is a serious trouble code that should be addressed immediately. Here are the possible triggers related to the code: 

  • Malfunctioning PCM
  • Faulty control module circuit
  • Fault in the data bus

What are the Common Symptoms of the U0100 Code? 

The most common symptom of this code is an activated check engine light. But sometimes you may not have any warning lights or you may have warning lights you don’t recognize, depending on the year model of the vehicle. U0100 code symptoms range from no symptom at all to a possibility that your vehicle will not be able to start or that the starter may not even engage. Once this happens, it is recommended to have your vehicle checked right away for proper diagnosis. 

How to Diagnose the U0100 Code 

Again, U0100 can be a very serious issue but it might not be. To diagnose this code properly, a technician will apply special procedures (usually involving an automotive oscilloscope) to pinpoint which module isn’t communicating and/or and what part of the bus is causing the problem.

Uxxx codes aren’t usually easy to troubleshoot or even sort out, and simply swapping parts based on what you think it might be can get you in a lot of trouble. Mostly, just checking fuses or doing a no-touch visual inspection for damaged, pinched, or chafing wiring is about as far as an untrained person can go with a Uxxx code without getting in deep water.

To be honest, where a Uxxx code is concerned, it is best to leave the job to a professional, but it’s your vehicle, your money, and your choice.

However, if you’re confident about your DIY auto repair skills regarding electronic troubleshooting, here are videos you can watch to help you learn more about the troubleshooting process:

How to Fix the U0100 Code 

U0100 is a generic trouble code, which means it can be seen in various makes and models. However, take note that there is no universal fix for the code. This means that the exact fix will still depend on your vehicle’s make and model. For example, repair steps for a U0100 in Honda may be different from a U0100 in a Ford vehicle. 

If you’re unsure about anything, it is best to bring your vehicle or have it towed to your trusted auto repair shop.  If you’re confident you can fix it yourself, you can use online auto repair resources and guides to determine what solution applies to your vehicle’s specific make and model. You can also consider getting an ALLDATA single-vehicle subscription.

Lastly, don’t forget to check your vehicle’s manual before doing any repairs.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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