Introduction to the Concept of Automotive ECU Diagnosis Technology Mar 19, 2023

What is An ECU? Electronic Control Unit(ECU) Explained


Electronic Control Unit (ECU) is short for electronic control unit. From the point of use is the car special microcomputer. The function of modern automobile is more and more rich. Every subsystem of automobile needs powerful ECU to control the work of this system. For example, the transmission ECU adjusts the appropriate gear according to the current speed and speed, and the air conditioning ECU automatically adjusts the temperature according to the current temperature.


Logically speaking, the functions of ECUs can be divided into three types:

1. Logical control function. Simply put, the ECU takes an input signal from a sensor or bus, performs a series of calculations, and outputs the action through an actuator. This is the most basic and primitive function of ECUs.

2. Bus function. Ecus do not exist in isolation in the on-board network. Information needs to be exchanged between ECUs. For example, the meter needs the speed signal from the engine to correctly display the current speed. Bus function refers to the ECUs in the on-board network to exchange data information function. Current on-board bus technologies include LIN, CAN, Flexray, MOST, Ethernet, etc. These bus technologies have different transmission rates, implementation methods, and prices, and are suitable for different scenarios.


3.Diagnostic function. This is the main topic of this column. Diagnosis, literally, is to detect a fault, to find a fault. Once the vehicle is assembled, it is not easy for us to know all kinds of information hidden in the vehicle body. For example, there is something wrong with the refrigeration effect of our car's air conditioning. After arriving at the 4S shop, do we need to disassemble the air conditioning system to check? At this time diagnostic technology comes in handy, we can use the diagnostic instrument to read the cause of the fault from the air conditioning ECUs, so as to make more targeted maintenance. Originally conceived as a response to regulatory requirements for emissions testing, diagnostics have evolved to mean much more broadly. Diagnosis is almost the only way that the device outside the car can get information about the car. In summary, diagnostics is the technology that enables the exchange of data information between the external device and the vehicle ECUs.


4.Diagnostic applications include:

1). Read the ECU fault code in after-sales service, accurately locate faults, and save maintenance hours.

2). Conduct coding, parameter rewriting, terminal component detection and other operations for models with different configurations before the vehicle comes off the assembly line.

3). Update the ECU software during the development phase.

Diagnostics is a communication process that is also implemented based on the OSI Layer 7 protocol. The following is the ISO specification used for diagnostics based on the high-speed CAN bus.


ECU wiring harness 

The full name of the UDS is Unified Diagnostic Services. The ISO standard number is 14229, where 14229-3 defines the implementation of the UDS on the CAN bus. UDS is an application-layer protocol that defines all diagnostic commands, such as the format of diagnostic requests and responses.

OBD is a protocol mandated by laws and regulations. It is mainly used in emissions-related systems, such as engines and transmissions. It is also an application layer protocol that defines the diagnostic commands related to emissions testing required by regulations.

ISO 14229-2 defines services for the application layer, The UDS provides independence from the transport layer and network layer (ISO 15765-2DoCAN, ISO 10681-2 Communication on FlexRay, ISO 13400 DoIP, ISO 14230-2 DoK-Line, etc.).

The transport layer and network layer are realized by ISO15765-2, whose main function is to provide segmentation, flow control, error detection and other functions for data packets.

The data link layer and physical layer are the requirements of CAN bus realized by ISO11898. This is just one possibility, but Ethernet technology can also be used for diagnostic communication, as defined in ISO13400. Regardless of the underlying bus technology, the ultimate goal is to transfer diagnostic commands and responses between the diagnostics and the ECUS.

The diagnostic communication process is very simple from the point of view of the diagnostic instrument user. The diagnostic instrument makes a request and the ECU gives a response. In this communication process, the diagnostic instrument and the ECU play the roles of client and server respectively in the computer network communication.

Auto Cable Assembly 

Diagnostics is not just about the narrow concept of servicing the car. It is a summary of the communication process between the external equipment and the car ECUs. It is the most important means for us to obtain the internal information of the car and configure the car. Diagnostic communication also follows the OSI seven-layer model, and each level uses the corresponding ISO specification, which we will have the opportunity to discuss in more detail later.


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