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U0100 Fault Code

You can find information about the U0100 fault code on this page.

We have given it a seriousness of 9/10 with 1 being the least serious and 10 being the worst.

U0100 Short Description

"Lost Communication With ECM/PCM "A""

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Description Icon Long Description

Fault code U0100 is a communication fault code that is logged when a loss of communications has been detected from the powertrain control module (PCM), also called the engine control module (ECM).

Modern vehicles are loaded with a number of onboard computers that have various functions. These are called electronic control modules (ECUs). The ECM or PCM is the primary one of these as it controls all of the engine and its auxiliary systems.

A loss of communication from the PCM will usually cause a massive drop in performance due to its importance. It is so important that it may cause a vehicle to be unable to operate at all.

Ford, Mazda, and Toyota are some of the most common manufacturers that are searched alongside this code.

Warnings Icon Other Signs of This Problem

As touched on earlier, this fault code will typically be accompanied by a number of significant problems.

The most obvious of these is often the presence of the Check Engine light on the dashboard. However, due to the communication loss, this may be erratic in nature. It is possible that the light does not come on at all.

Another common and very significant issue is that the vehicle may not start at all. This can be the result of vital systems not receiving commands from the PCM.

In addition to this, if the vehicle does start, these systems will not function properly. This will result in huge reductions in power, fuel economy, and ride smoothness.

There may also be other codes that indicate a loss of communication with the various control modules. These will be codes like U0101, U0102, up until U0115.

Broken Down Icon What Could Be The Problem?

The cause for a communication loss with one of the various control modules is typically the result of an electrical issue.

The most common cause is a faulty battery. This is because the ECUs require sufficient voltage to power them.

Electrical faults in the PCM circuit can be another likely cause. Damaged wires and loose connections are among the main issues.

Similarly, these electrical faults may be present in the CAN bus circuit. This can include open or short circuit conditions in either of the CAN bus wires. It is likely that there is either a short to ground or short to power.

Fixed Icon Fixes

Fixing this code will typically require some significant labor and electrical repairs. It is typically recommended to leave this type of issue to a certified technician that has experience with the make and model.

The battery needs to be checked for the correct voltage. If the battery is failing, it needs to be replaced.

The PCM itself should be checked next. If it can be determined that it is faulty, it should be replaced.

It is also important to check the connections to the PCM and its circuit. Any bent, loose, or damaged connections need to be fixed. This includes damaged or frayed wires and anything that could cause an open or short circuit.

The high-speed CAN bus network also needs to be checked for similar issues. A short-to-ground, and short-to-power all need to be addressed if they are present. The same goes for anything causing an open circuit.

Danger Icon Seriousness

Fault code U0100 is a serious code. The PCM is responsible for a huge number of critical systems throughout the vehicle, and a loss of communication at any time can cause any number of them to malfunction.

It is a good idea to have this code fixed immediately.

U0100 Image

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