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

The code U0100 means: Lost communication with ECM/PCM “A”. U0100 is a generic OBD-II code that indicates a problem with loss of communication between a controller and the computer.

The PCM is the powertrain control module. It’s a computer in your car that monitors the engine and controls its performance. U0100 is a general error code that indicates a network communication problem.

In this detailed guide, learn how to repair the code U0100 and the reasons for it.

PCM operation

The PCM It is part of the vehicle’s engine control unit (ECU). It houses the software that controls how the engine works.

The PCM is an essential component of the automobile’s engine management system, which is responsible for maximizing fuel efficiency and reducing emissions. It also helps your car start smoothly and run at its best performance level.

Every time you start your car, the PCM runs a series of checks to ensure the car’s systems are ready to go.

If there is a problem, he will let you know. You can check the PCM yourself by pressing the OBD (on-board diagnostics) button on your dashboard or by plugging an OBD scanner into your car’s data port.

One of the possible causes of loss of communication with the ECM PCM is if the powertrain control module (PCM) does not receive a signal from the engine coolant temperature sensor.

If this is the case, you should be able to diagnose it with a simple scan tool.

The engine coolant temperature sensor is located near the thermostat housing, so you may need to remove some components from the front of the engine, such as the radiator, to access it.

However, if this does not solve your problem, there are other possible causes for the loss of communication. This includes poor connection of the ECM or PCM, or both modules damaged.

What is Code U0100?

The code U0100 means that the powertrain control module (PCM) has detected a problem with the data bus communication system, or that it is not receiving signals from other modules on the data bus.

This type of error can be caused by a poor connection or wiring problem in the CAN-C data circuit. It can also be caused by a faulty ECM or a damaged body control module (BCM).

The reason the code is set is that the PCM (Powertrain Control Module) is not seeing the communication from the other module. This indicates that there is a problem with the wiring or modules involved.

The vehicle may shut down and not restart unless the issue causing the code is resolved. The entire system is connected to other modules of the car via a data system.

You may also be interested in P2610 OBD-II Error Code: ECM/PCM Internal Engine Shutdown Timer Performance

Causes of Code U0100

The code U0100 It has many probable causes. The problem may have started when the battery was disconnected for maintenance, or it could be a symptom of a faulty module.

  • Faulty engine control module
  • Bad connection with an engine control module
  • Faulty transmission control module
  • Defective connector on transmission or ECM connector pins do not align properly

To be safe, check the module’s wiring and connectors and make sure they are clean and free of corrosion.

In some rare cases, the U0100 code can be caused by incorrect data on the computer itself. In this case, you should try clearing the code and driving for a few days to see if it comes back.

If so, you will need to take your car to a mechanic to make sure it is safe to drive again.

Additionally, the code is often caused by a faulty connection between the powertrain control module (PCM) and the instrument panel cluster (IPC). This may be due to a loose connector or a buildup of corrosion.

The PCM is the central computer system that controls all of the vehicle’s engine components, including the engine itself, transmission, and brakes. The IPC is the on-board computer that controls everything related to your dashboard.

Symptoms of Code U0100

If your car shows a U0100 code it is because there is no communication between the engine control module and the transmission control module.

This could be due to a problem with the wiring harnesses or connectors. The cables in question should be inspected for damage and replaced if necessary.

These are some of the symptoms:

  • The engine may stall or not start at all.
  • The engine may run rough or misfire.
  • You may experience increased fuel consumption.
  • The Check Engine light comes on.
  • Irregular idle.

The most common method to clear this code is to disconnect the batteries from both modules for 10 minutes, at which time you will reconnect them and start your vehicle to see if the error message is cleared.

If this doesn’t work, you may need to use a scan tool to read any diagnostic trouble codes stored in the ECM’s memory.

How to Diagnose Code U0100?

If your vehicle develops a code U0100, you should diagnose it as soon as possible. This can be done quite easily by connecting a diagnostic scanner to the car’s OBD-II port and reading the data stream.

The most common reason for this code is simply that there has been a failure in one of the car’s computer modules and it has lost connection to the central ECU.

The problem is almost always a problem with the wiring or an electrical component such as the battery, ignition switch, or relay.

Here are some things to check:

1. Test the ECM computer

To test the ECM computer, you need to follow the steps below:

  1. Turn the ignition key to the “position”OFF“.
  2. Plug your vehicle’s diagnostic tool into the data link connector (DLC) on your computer.
  3. Turn on the diagnostic tool.
  4. Choose “Read codes“. The diagnostic tool will display any trouble codes stored in the ECM computer memory.
  5. Write down the codes displayed and then select “Delete codes“.
  6. Disconnect your car’s DLC diagnostic tool, wait two minutes, and then turn on your vehicle to reset the ECM computer memory.

2. Check the battery status

Check the battery voltage under load conditions and if it drops below 10 volts, it is not providing enough power to keep all the computer modules running.

You must proceed to change it.

3. Check the wiring and its elements

If a relay was involved in the communication loss, try replacing it. If this solves the problem, you may need to replace all relays related to the fault.

Check for loose or broken wires in the harnesses that run from the computer modules that transmit the U0100 codes to their respective control units.

If none of these measures work, your best option would be to take your vehicle to a mechanic who specializes in diagnosing vehicle electronic systems.

How to Repair Code U0100?

If the U0100 code keeps appearing on your car, you can fix it by following these steps:

1. First erase all codes

The basic step in the steps to fix the U0100 code is to clear the codes present. Error code U0100 is not that common. The consequence could possibly be a discharged battery.

Clear the code to get started and see if it appears again. When the code returns, perform a visual check. Codes U0100 to U0300 are associated with failed modular communication.

Network communication codes are also related to other U codes.

2. Check the wiring

The electrical cable is made up of braided copper wires, of cross section, with particular plastic insulation. The wiring is collected and placed in particular channels in the body.

Electrical tape, clamps, and clips are used to join electrical wires together. You should first look for corroded terminals during the wiring test.

The color of the car wiring for convenience and difference varies depending on the device used. Color coding is done to facilitate troubleshooting and easy maintenance of vehicles.

Use a multimeter to check the various wiring values.

3. Check the ECM/PCM

We have a powered side of the battery, a device that we control, or a sensor that we read a ground and value from. The ground adjusts the circuit back to the battery.

When connected to multiple ground wires from the same 2 points, the current is shared between the two alternating ground channels. The ECU shares some current from the starter motor in other words.

In other words, check the reasons for the ECU as it may be the whole problem.

4. Check the battery

The ECU and PCM rely heavily on the battery to function properly. The CAN bus system particularly requires a voltage so that it can communicate correctly.

If your car has a faulty battery, then the CAN bus system will not be able to work properly and it is very likely that you will get this error.

So in this case, getting a new battery or recharging the old battery, if it is still fine, are the only options to fix this error.

Check that you have a good connection between the battery and the negative terminal. The battery must be charged and the terminals must be clean. If they are not, clean them with a brush and reconnect them.

5. Check the fuses

If that doesn’t work, check the fuse box for blown fuses. Replace any blown fuses and see if this has resolved the problem.

If cleaning the battery terminals or replacing the fuses doesn’t clear the U0100 code, take your car to a mechanic’s shop for a more in-depth diagnostic test.

How Serious Is Code U0100?

A vehicle’s computer has many different sensors that monitor various aspects of its operation. When any of these sensors detect a problem, they will trigger an error code.

The most common cause of this type of error code is a faulty sensor or because the wiring has been damaged over time.

Generally, if you see this code, it means that there is a problem with your vehicle’s communication network and it is not safe for you to drive in these conditions. In many cases it will even be difficult to start the car.

We can say then that the U0100 code is of moderate to high severity.

Can I Fix Code U0100 Myself?

For a U0100 code, repairs are best left to the professionals. Because the U0100 code indicates that there is a loss of communication, but does not point to the direct cause, the problem must first be diagnosed before it can be fixed.

If your car displays a U0100 code, you can rule out some issues that may be causing the code. Make sure your car battery is charged and check for broken cables or damaged components inside your car.

You may be able to see if the cables are disconnected, but if the problem lies with the PCM itself, more complicated repairs will be required.


You may also be interested in Error Code P1607 – Rationality of the internal PCM shutdown timer

Error code U0100 It can leave you stranded if not attended to in time. By learning how to fix code U0100 and the associated symptoms, you can avoid a repair nightmare. Don’t ignore this problem and fix it as soon as possible.

Also, avoid installing any third-party accessories on modern cars if they are not specifically designed for your car.

If you want to know other articles similar to Code U0100. Lost Communication with ECM/PCM “A” you can visit the category Fault Codes.


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