The check engine light is on, and all of a sudden you are noticing a lot of major problems with your car. Fuel consumption significantly rises, the engine behaves erratically, and it is hard to turn it on to begin with. In some cases, you are unable to start the car. You hook up the car to the OBD-II scanner and get a U0100 code.

A U0100 code comes with the message “Lost communication with ECM/PCM”. That means that the car’s high-speed controller area network bus (CAN C) can’t receive any signals or messages from the engine control module (ECM) and/or powertrain control module (PCM). The problem is usually in the network bus, the PCM, or the wiring harness.

This is a very serious issue that needs to be resolved as soon as possible. Driving with this error will put a strain on the engine, and can cause multiple problems. Stay with us, while we go through all the aspects of this error message and ways to resolve it.

U0100 Code: Lost communication with ECM/PCM

Decoding the U0100 Code – Understanding the Basics

Modern cars have several sensors and modules that allow the car to work smoothly. The most important are the powertrain control module (PCM) and the electronic control module (ECM).

• The powertrain control module is responsible for maintaining the proper air-to-fuel ratio. That ratio needs to be 14.7:1 in favor of air to facilitate proper engine function
• The electronic or engine control module gathers information on how is the engine running. It is responsible for getting enough fuel to the engine and at the same time lowering the emission rates.

The final piece of the puzzle is the controller area network bus CAN-C. It maintains the communication between these modules. The CAN-C is also communicating with other systems, like with transmission control module and the ABS.

The underlying issue for the U0100 can be a faulty PCM or ECM, or a CAN-C controller is in trouble. In case the controller is the culprit, that would resonate with all the systems in the car. That network resembles the nervous system in humans and animals. Any part of the system can be affected by the faulty CAN-C controller and will cause communication issues.

The Importance of OBD-II Codes in Modern Vehicle Diagnostics

The OBD-II diagnostic system made diagnosing car problems much easier. All vehicles in use in the US after 1996. have an OBD-II system implemented. That means that anybody with an OBD-II scanner can check the car’s computer for an error.

Every system in the car has its share of diagnostic codes. If the person doesn’t know what the error means, they can find a manual online or check with a local mechanic. That way it is easier to narrow down and speed up the diagnostic process.

What Does OBD2 Code U0100 Mean

The U0100 code means that there is a problem or a lack of communication between the PCM/ECU and the CAN-C controller. This is a communication fault code that will cause a lot of trouble for all the modules and sensors affected by it. 

Causes and Symptoms of the U0100 Code

Common symptoms of the U0100 code are easy to notice and will make driving difficult. The first symptom is a check engine light. Soon after these engine performance issues will step in:

• Drop in fuel economy. The problem on the CAN-Bus network will lead to bad information getting to the PCM and ECU. That will result in bad fuel consumption and raise fuel consumption
• The engine unexpectedly stalls. If the fuel mix is not good that will cause the engine to stall. The worst part is that the driver cannot know when the stalling will happen
• The loss of power. Fuel combustion is the process that is moving the car. When there is a problem with a bad engine control module, or with the communication network, the engine cannot work properly. One of the symptoms is low engine power. It cannot accelerate properly and will have poor performance
• Erratic shifting gears. If the communication issue is serious, that will affect the transmission control module. It is responsible for shifting the gears at the right time. If it cannot communicate and get precise information, that will trigger a lot of problems with shifting gears
• Increase of dangerous emissions. The car has several emission control systems and sensors. They control and minimize the amount of bad emissions ending up in the atmosphere. If those systems don’t work properly, then the emissions will rise. Also, if the combustion mix is too rich that will cause an increase in bad emissions
• Inability to start the car. If the problems are affecting multiple systems that can prevent the car from starting in the first place. The cause for this can also be a bad battery.

Common Causes of the U0100 Code – Are You at Risk?

Before you start worrying about this problem, you should know what can cause an error code U0100. Sometimes the issue is easy to fix, but there are times when you will have to splash out a sizeable amount of money. The usual suspects for a U0100 code are:

• The problem caused by the CAN-C bus wiring fault
• Faulty wiring on one or more modules
• Problems with one or more CAN-C bus network connectors
• Wiring problem on one or more Body Control Modules or a Totally Integrated Power Module
• Problem with the PCM or its wiring
• Problematic car’s data bus
• Issue with a depleted, corroded, or weak battery.

Recognizable Symptoms: Know Your Vehicle’s Distress Signals

The first thing that you will notice is an illuminated check engine light. With it will come different performance issues. What you will experience depends on the car and the specific cause of this diagnostic trouble code. But, one or more problems from this list will hit you:

• Illuminated check engine light
• Problems with accelerating
• Stalling issues
• One or more safety systems (ABS, ESP, etc.) are not working
• Increased fuel consumption
• The transmission often goes into the wrong gear.

Demystifying the U0100 Code with Professional Diagnostic Steps

Diagnosing the U0100 error code should start with obvious and simple tests and fixes. If those don’t show any improvement, then it is better to contact the mechanic.

Preliminary Checks Before Proceeding

• Inspect the car with a code reader. Check will there are any additional error codes. If there is, then you should address them first. Those errors could be the real reason for the U0100 code. The most frequent additional fault codes are P0606, U0101, U0073, U0001, and U1000. Delete the codes, and do a test drive. Monitor what errors will turn up, and when
• Check the state of the battery. Corroded connectors or a weak battery will cause power supply issues. Visit a mechanic and check the battery voltage condition. When there is not enough power, then the modules and sensors cannot work properly

Diagnostic Procedure to Identify the U0100 Code Issues

If preliminary checks didn’t yield results there are a couple of things you can try:

• Check the wire harness. If there are any burned, frayed, exposed, or loose wires you will need to either fix or replace them 
• Inspect the CAN-C bus and its connectors. Check our the connectors are okay with a multimeter
• Check the positive and ground wires of the PCM. Find a diagram for the PCM and find the right wires. Use a multimeter to test them
• Visually check the PCM wiring and connectors. Sometimes a burned or corroded wire or a loose connector can cause this problem.

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Comprehensive Guide to U0100 Code Repair and Prevention

Expert Tips on Fixing U0100 Code Issues

Experts know when they run out of usual things to check and test, they turn to other electrical components. Sometimes a specific blown fuse can result in a U0100 code and a series of other problems. Also, a troubled alternator can be the cause of this issue.

Preventive Measures to Avoid the U0100 Code

The best way to prevent the U0100 code from triggering in the first place is to regularly maintain the vehicle. Follow through on all maintenance cycles. Also, talk with the mechanic and tell him to visually check the wiring and the state of the modules and sensors.

Electrical components are susceptible to water damage. Try to avoid exposing the wiring and the electrical systems to excessive water. If there are problems with the wiring or sensors, replace them as soon as possible. Leaving these problems unattended will result in new, bigger issues.

It is also a good idea to check the battery from time to time. If you don’t want to waste time checking the battery voltage, you can do a test in almost all car parts shops. Inspect the connections for corrosion, and if there is any, use a battery terminal cleaner to remove it.

Most communication network errors will trigger a warning light. But, just in case, from time to time check the car with the OBD-II scanner. That way you will know if the system signaling any errors.

FAQs about the U0100 Code

What is the Cost to Resolve the U0100 Code Issue?

The cost to resolve the issue with the U0100 code depends on the car and the cause. The most usual suspects and the cost to fix them are:

• Faulty wiring harness. Replacing an entire wiring harness can cost anywhere from $1,700 to $2,200. The price of labor varies between $400 and $500, while the harness costs between $1,300 and $1,700
• Broken PCM. If you need to replace the PCM you can expect to pay between $500 and $1,500 for the new module. Labor cost depends on the mechanic and the model, and is in the region of $100-$150
• Dead battery. The price for a new battery can significantly differ on the type and manufacturer of the battery. It can go as low as $60 but can rise to a hefty $300. 

Can I Drive My Car with the U0100 Code?

No, you should not drive with the U0100 code. The U0100 code can cause unexpected problems like stalling and loss of power. You can never know when those can happen. Also, you can end up with a car sitting in the parking lot, or a driveway, unable to start. Once you detect this code, do a basic checkup, and then visit the mechanic.

How Do You Test PCM Power?

There are several tests that you can do to check the PCM power. The best thing to do is to start by testing the voltage of the battery in the PCM. To do so you will need a multimeter.

1. Set the multimeter to measure DC volts
2. Connect the multimeter probes to the PCM terminals
3. The readout should show around 12V
4. If the readout is less than 10V check for loose connectors or other power issues

If this test doesn’t show anything strange, do a diode test of the PCM.

1. Connect the multimeter probes to two pins on the PCM circuit board
2. The reading should say “Open Circuit”. If it doesn’t then the PCM is faulty

If the PCM still doesn’t show any issues, you can check the resistance of every component that is connected to it.

1. Set the multimeter to measure resistance (ohms)
2. Check the manual to see what resistance each component should have
3. Measure each component individually, and compare the measured values with those in the manual
4. If any component shows strange resistance values, additionally check them. If they are faulty replace them

As an ultimate test, check the voltage of every component externally connected to the PCM.

1. Set the multimeter to measure DC volts
2. Disconnect one component at a time, and check them with a multimeter
3. Have a manual at hand to compare the values with those recommended by the manufacturer
4. Replace any component with a faulty readout.

You can also do a voltage drop test for the power circuit of the PCM.

1. Disconnect the negative battery terminal before starting the test
2. Set the multimeter to measure DC volts
3. Connect the probes to the positive and negative terminals of the power circuit
4. Turn the ignition on, but do not turn the engine on. Turn on all the electrical components, like lights, the fan
5. Record the measured values. The normal range is between 11 and 14 volts
6. If the readout is less than 11 volts, then the issue can be the PCM, the alternator, or the fuse box
7. Turn the engine on and check the multimeter readout. When all electrical components are on, the readout should show 13-14 volts

If the tests show normal results, then the problem is not in the PCM or other major electrical components.

What Causes ECM Communication Failure?

The ECM circuit board has 4 types of circuits, and they are responsible for communication between modules.

1. Circuits and components that process and compute information. They are also storing the firmware that the module is using
2. Communication circuits. These circuits allow communication with other modules in the car
3. The circuit that receives information and generates voltage. Those, among others, include a coolant temperature sensor, mass airflow sensors 
4. Circuits that drive components. Among those are ignition coils, injectors, and relays.

All these circuits are prone to damage from heat and vibration. The damage can happen to the microchips, transistors, or other circuit components. This damage can, and usually will cause communication failure.

How Long Does It Take to Fix the U0100 Code?

It takes about an hour for an experienced mechanic can diagnose a U0100 code. That includes testing with an OBD-II scanner and the multimeter. If the cause is unusual the diagnosing and testing may take a bit longer. How much time will take to fix the issue, depends on the issue that is diagnosed.

Need More Help with a U0100 Code?

Diagnosing and fixing a U0100 code can be tricky for an amateur mechanic. It requires more than basic car electronic experience and an understanding of electronic diagrams. If you are experiencing serious car issues, and you do not know how to address the problem, visit a mechanic.

We do not recommend driving with this error code. Even if the symptoms are minor, they can escalate quickly. The car can malfunction all of a sudden, leaving you in the middle of the road. Do not leave this problem unattended, and get a fix as soon as possible.