U0100 – Data bus: engine control module (ECM) A – no communication

By Randy Worner (Contact Me)
Last Updated 2016-06-21
ASE/ACA World Class Technician

Table of Contents

1. What Does Code U0100 Mean?
2. What are the common causes of code U0100?
3. What are the symptoms of code U0100?
4. How do you troubleshoot code U0100?
5. Get Help with U0100

What Does Code U0100 Mean?

This code is concerned with the communications circuit between the engine control module and other control modules throughout the vehicle. This communications circuit is most often referred to as Controller Area Network bus communications, or simply put, CAN bus. Without this CAN bus, control modules cannot exchange information, and your scan tool may not be able to communicate with the engine control module, depending on which circuit is affected.

The CAN bus communication system is wired throughout the vehicle, however for this code we are going to concern ourselves with CAN C, or High Speed CAN, which may also be referred to as under-hood or under-vehicle CAN bus, as the modules that typically communicate on this network can be found under-hood or under-car.

What is causing this code to set is one or more modules have the task of communicating with the ECM/PCM on CAN C to make sure that it is able to communicate or “check in”. If unable to check in, one or more modules can set the U0100 code to indicate that at some point in time it lost the ability to talk to the ECM/PCM. This code is usually a “memory” code, or something that occurred in the past and may be nothing to worry about. If the code continues to come back, further checks will be needed to find out why the modules are losing communication with the ECM/PCM.

What are the common causes of code U0100?

• Open in the power supply to the ECM
• Open in the ground supply to the ECM
• Open in the CAN bus + circuit
• Open in the CAN bus – circuit
• Faulty ECM – rarely

What are the symptoms of code U0100?

• Malfunction Indicator Light (MIL) “On”
• Lack of power
• Poor fuel economy
• All engine related indicators (lights) “on” in the instrument cluster
• All engine related gauges inoperative in the instrument cluster
• Possibly a no-crank, no-start

How do you troubleshoot code U0100?

Before tearing into body panels a good practice to get into is to always check for technical service bulletins (TSB) for your particular vehicle. The vehicle manufacturer may have a known issue for the particular problem you are dealing with on your vehicle and it pays to check it out before you head down the wrong path.

Next, see if you are able to access fault codes. If not, the High Speed CAN bus will need to be diagnosed. If you are able to access fault codes, note if there are any other diagnostic fault codes. If any of them are module communication related, depending on the code, you may need to diagnose them first. Misdiagnosis has been known to occur if a technician diagnoses this code before any other module communication related system codes have been thoroughly diagnosed.

If your scan tool can access fault codes and the only one you retrieve from other modules is the U0100, try to access the ECM/PCM. If you can access codes from the ECM/PCM, then the U0100 code is either intermittent or a memory code. If unable to access codes for the ECM/PCM, then the U0100 code that the other modules are setting is active, and the problem is there now.

The most common failure is loss of power or ground.

Check all fuses that power up the ECM/PCM on this vehicle. Check all grounds for the ECM/PCM. Locate where the ground attaching points are on the vehicle and make sure that these connections are clean and tight. If you have to, take them off, get a small wire bristle brush and baking soda/water solution and clean each one, both the connector and where it connects.

After checking powers and grounds, if any repairs were made, clear the diagnostic trouble codes from memory, and see if the U0100 code returns or if communication with the ECM/PCM is re-established. If the code does not return or communication is re-established, then the fuses/connections were most likely your problem.

If the code returns, you will need a digital volt ohm meter (DVOM). Set the meter up for ohms. Place one lead of the ohmmeter on terminal 6 of the datalink connector (connector your scan tool hooks up to) and the other lead of the ohmmeter to pin 14. You should get a reading of 60 ohms +/- 1 ohm. If you get a reading of 120 ohms, then the High Speed CAN (CAN C) bus circuit is open, either on the Bus + circuit, the Bus – circuit or internal to the ECM/PCM. (This particular step applies to most but not all systems.)

Next, locate the CAN C bus communication connections on your particular vehicle, most importantly the ECM/PCM connector. Disconnect the negative battery cable before unplugging the connector at a given control module. Once located, visually inspect the connectors and wiring. Look for chafing, rubbing, bare wires, burn spots or melted plastic. Pull the connectors apart and carefully inspect the terminals (the metal parts) inside the connectors. See if they look corroded or burnt. You can get some electrical contact cleaner at any parts store if cleaning of the terminals is needed. If this is not possible, find some rubbing alcohol and a light plastic bristle brush to clean them with. Afterwards let them air dry, get some dielectric silicone compound (same stuff they use for light bulb sockets and spark plug wires) and put some where the terminals come into contact.

If communication is still not possible, or you were unable to clear the module communication related fault codes, the only thing left that can be done is to seek assistance from a trained automotive diagnostician.

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