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Code U0100 (symptoms, causes and how to fix it)


As drivers know, a sudden turning on of a vehicle’s check lights is usually an indication of one type or another. In fact, a check engine light serves as a visual signal, signaling the storage of one or more fault codes by a vehicle’s PCM.

These error codes often identify the vehicle system that has been affected by a particular fault, providing a very general description of the problem in question.

However, there are also rare cases where a check engine light indicates that one or more vehicle control modules are unable to communicate within the data bus system itself.

This situation can best be illustrated by analyzing the diagnostic error code U0100. This trouble code describes a condition in which a vehicle’s PCM is unable to receive or transmit data to and from another OBD-II compatible module. This has the potential to cause a multitude of problems, varying greatly in severity.

Read on to learn more about the U0100 diagnostic error code and how to resolve these issues, should you encounter them in the future.

What does code U0100 mean?

OBD-II Trouble Code U0100 Description

Loss of communication with ECM/PCM “A”

A U0100 diagnostic error code indicates a loss of communication between a vehicle’s PCM/ECM and one or more corresponding modules within the CAN-bus network. However, to fully understand the severity of this situation, we must first analyze the operation of the modern CAN-bus network itself.

Today’s vehicles rely on the use of a highly sophisticated network of computing modules and sensors to facilitate their smooth operation. A single vehicle can contain many control modules, all of which must be able to communicate with each other in real time.

This live communication allows multiple vehicle systems to operate seamlessly. For example, a vehicle’s transmission control module depends on data from the ECM/PCM to determine the correct shift points. Likewise, modern vehicle stability systems require access to ECM/PCM data when attempting to break a car or truck from a skid.

In the case of DTC U0100, communication within the CAN bus network was interrupted, affecting critical vehicle functions. As a result, the operation of the vehicle is seriously hindered or completely impossible.

See also: code P0606, code U0001, code U1000

Code U0100 symptoms

check engine light

The U0100 diagnostic trouble code is usually accompanied by some additional symptoms. However, most of these symptoms are quite serious and can be a big obstacle.

These are some of the most common symptoms of DTC U0100.

u0100 reason codes

PCM vs ECM vs ECU vs TCM

The U0100 diagnostic trouble code can be caused by a number of individual problems, most of which vary greatly in severity. Understanding each of the possible causes of DTC U0100 can be helpful when trying to diagnose the problem at hand.

These are some of the most common causes of DTC U0100.

  • Defective ECM, TCM, or other network modules
  • “Open” wiring in the CAN-bus network
  • Grounded or shorted wiring on the CAN bus network
  • The contact failure is connected to one or more CAN-bus network connectors.

Is code U0100 serious?

The diagnostic trouble code is usually considered U0100 an extremely serious character. This is because such a condition can cause a vehicle to stop randomly, or can prevent a vehicle from starting, leaving the unfortunate motorist stranded.

In most cases, the root cause of DTC U0100 will require immediate diagnosis and repair as the vehicle’s drivability will be severely affected. If a problem of this nature arises, seemingly to resolve itself, do not lull yourself into a false sense of security. This problem will surely occur again, for example, when you least expect it.

In any case, the root cause of DTC U0100 should be diagnosed and repaired as soon as possible. This avoids the risk of stagnation or dangerous terrain. If you are not comfortable addressing these concerns yourself, schedule an appointment with a trusted service center as soon as possible.

How to fix code U0100

The following steps can be used to diagnose and repair the root cause of the U0100 vehicle trouble code. As always, you should consult a factory-specific service manual for your vehicle make and model before attempting such a repair.

#1: Check for additional DTCs

using an OBD2 scanner

Before starting the diagnostic process, use a quality scan tool to check for additional error codes. If such error codes are present, diagnose them thoroughly before continuing.

#2 – Inspect the PCM circuit wiring

Begin the diagnostic process by carefully inspecting the vehicle’s wiring harness, in relation to the PCM itself. Check for broken or damaged wires or any wiring that appears to be corroded.

#3 – Check PCM Connectors

Next, check all the connectors along the body of your vehicle’s PCM. Make sure that all cables are secure in their respective terminals and that there is no obvious damage related to the pins.

Likewise, each connector should also be checked for signs of corrosion. Any such problems must be corrected before continuing.

#4 – Check Battery Voltage

battery voltage

As simple as it sounds, checking the vehicle’s battery voltage is also extremely important when combating U0100-related issues. At rest, a fully charged battery should have a charge of about 12.6 volts.

#5 – Inspect PCM Power Supply Positive/Ground

Using a model-specific wiring diagram, locate the positive and ground supplies for your vehicle’s PCM. Use a digital multimeter to check for positive and ground signals, with the vehicle’s ignition in the “on” position.

#6 – Analyze the PCM

If the source of the U0100 diagnostic trouble code was not found in steps 1 through 6, your vehicle’s PCM may have failed. In this case, a replacement will be required.

Many PCMs must also be “sprinkled” with the manufacturer’s software to facilitate proper use. This usually requires a trip to your local dealer.



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