U0100 - How To Repair: Meaning, Causes, Symptoms, Fixes

U0100 Code Definition

Lost Communication With ECM/PCM “A”

What Does the Engine Code U0100 Mean?

Character "U" in the first position of Diagnostic Trouble Code (DTC) represents the vehicle’s network (onboard computers and related systems), "0" in the second position means that this is a generic OBD-II (OBD2) DTC. "1" in the third character position in a DTC indicates that the network communication is experiencing a malfunction. The last two characters "00" is DTC number. OBD2 Diagnostic Trouble Code U0100 means that Lost Communication With ECM/PCM has been detected.

What Is the Cause of U0100?

There can be several damages/failures causing the underlying problem of the OBD2 DTC U0100. To find the root cause of U0100, unlock the “Smart Mechanic" feature in the Carly app.

What Are the Symptoms of the Code U0100?

Symptoms of a faulty bus system/booting errors

Sporadic errors during driving or sporadically illuminated error lamps

Failure of various systems (mostly in the area of audio/navi functions)

Error stored in the error memory without recognizable effects

Intermittent shutdown, non-responsive control units

How To Diagnose and Clear the Code U0100?

Performing a diagnostics check with a diagnostic scanner helps you identify a particular problem area that is outside the normal range and shows where a fault might be occurring. This helps you to detect errors before they cause more damage to your vehicle. In comparison to generic OBD devices that only provide OBD-level engine-related data, Carly can run advanced manufacturer-level diagnostics, check all your ECUs in your car (like ABS, airbag, multimedia) and get deeper insights. You can find additional information about the trouble code and understand the severity of the problem. With the new Smart Mechanic feature, you get additional professional insights and tips from expert mechanics. You are able to check components and their mechanics, find detailed repair tips, and learn about potential future damage. After you have fixed the problem you can easily clear the fault codes with Carly, too. Also, the diagnostic results are safely stored inside the app or as convenient PDF reports you can share with your friends or a mechanic.

Common Mistakes when diagnosing the U0100 code

The most common mistake is to look and try to fix each individual OBD2 (OBD-II) code without having a holistic understanding and approach. Understanding the interrelationship of different systems is crucial to determining the root cause of the problem and finding the right solution. The new Carly Smart Mechanic feature provides a look at the whole system, not just one individual code. Enable Smart Mechanic in your Carly app and get a holistic solution-focused approach when diagnosing the code U0100.

How Serious Is the Code U0100?

For now, this issue is not serious. Carly recommends you to check this issue on a regular basis and check if it gets worse later on.

How to fix the U0100 Code

U0100 is a generic fault code but the repair steps may vary depending on the root cause and the make/model of your vehicle. Get the Smart Mechanic in your Carly app to learn more and receive guidance.

Learn more about the U0100 Code

Functionality of CAN-BUS/bus systems

Modern passenger cars are nowadays equipped with a large number of electronic systems. Commonly, modern luxury/mid-range vehicles to have up to 50 control units. Most of these electronic systems also have to communicate with each other. Due to the large number of control units, conventional wiring (a separate cable harness for each control unit) is hardly possible, as this would result in gigantically large and long cable harnesses. Since installation space in a vehicle is limited, cables (copper) are becoming increasingly expensive, and weight is in focus, another solution had to be created. To replace the conventional "wiring harness", the individual lines of the control units were networked directly with each other, which is known as a bus system. These are line systems for data transmission between the various components/control units. In the automotive sector, the CAN bus (Controller Area Network) has become the most common bus system. The CAN bus is a bidirectional bus system with a data transmission speed of up to 1 Mbit/s, which enables serial data exchange in half-duplex mode between control units (up to 128). The engine control unit, for example, can communicate with the transmission control unit via a twisted pair cable (only two lines are required, CAN-HIGH and CAN-LOW). A third CAN-GND line (ground) is optional and, if desired, is usually provided in vehicle construction by connecting the control unit directly to the body (vehicle ground). Thus, normally there are only two twisted visible lines. The signal voltages on HIGH and LOW are in opposite directions (summing up to zero), which are therefore redundant. In a Low-speed CAN system, one of these lines can fail if a ground line (body connection) is present. This is not possible with high-speed CAN systems due to the high data throughput rate. The achievable data transmission speed depends primarily on conditions such as line length, bus utilization or transfer errors due to interference.

Differences between the common CAN systems

Cars on the roads usually use two CAN classes. 1) CAN B = Comfort CAN (KCAN) with speed up to 125 kBit/s (ISO 11989-3)Advantage - Robust, fault-tolerant and fail-safe (max. cable length 500m) (in case of wire break/short circuit can be switched to ON-WIRE operation) Disadvantage - Slower (low-speed CAN) 2) CAN C = Drive CAN (PTCAN) with speed up to 1 Mbit/s (ISO 11989-2) Advantage - Fast (high-speed CAN) and "real time" capable Disadvantage - More vulnerable, not fail-safe (max. line length only 25m, at 500 kBit/s = 125m) (No ONE-WIRE OPERATION possible) Each control unit is connected to the BUS via a CAN interface (BUS controller + BUS transceiver) and first checks whether the data packets sent via the BUS are of significance to the control unit. If this is not the case, the message is discarded. If several control units try to send information at the same time, it is checked which message has the highest priority. Messages with highest priority are sent first, the other messages follow in order of priority as soon as the bus is free again. The CAN-BUS can also detect errors with transferring signals and is repeating them accordingly.

*Every car is unique. The Carly Features that your car supports depend on the model, build year, hardware, and software in the car. Plug the scanner in the OBD2 port, connect to the app, run your first diagnostics, and check which features are available for your car. Also, keep in mind that any information provided on this website is for informational purposes only and should be used at your own risk. Mycarly.com is not responsible for any errors or omissions, or for the results stemming from the use of this information.