Unified Diagnostic Services (UDS)

What is “UDS”? UDS is an automotive standard based on the ISO-Standard 14229. UDS is basically a diagnostic protocol based around services. Each service has an identifier that represents a task or job.

In CAN communication, you need to know the ID of the module. This ID value is normally a HEX value such as 0x7E0. The Hex value is known as the “Request ID” of the module. this is like calling some one by name when you want to talk to them as the name is 0x7E0 in this example. (0x before any number means that the value is in HEX). The module has a response ID that you can see on the CAN BUS trace. the response ID is a value of 8 higher than the request ID for example if the HEX ID is 0x7E8 when you convert the hex value to decimal you get 2016 if you add 8 to that number you get 2024 then when you convert this value back to HEX your response value or ID is 0x7E8

Service $10 “Diagnostic Session Control” (Allowed During Default Session) – This service is used to control what session the module’s in. For example when the ECU is running normally this is said to be in the DEFAULT session HEX value 0x01. When Programming also known as re-flashing the module it’s required to be in the Programming session HEX value 0x02. When performing a take over of the module (performing a short term adjustment) for example forcing the engine fan to run or controlling a window to roll up or down. This requires an EXTENDED session HEX 0x03. On the CAN BUS while using a CAN trace tool the message will look like this 762 8 02 10 03 55 55 55 55 55. This is known as sending out a request. In the massage you can see the Service 10 and the 03 then all the padding bytes 55 or AA is the standard for padding. The Response Can message will look like this 76A 8 02 50 03  you can see the response ID and the Positive response 50 to our service $10 request. A positive response rules are to add 40 to your request. This is why you show 50. If you received a Negative response this is displayed as a 7F. The code 7F is the value for a negative response. 7F 10 13 would mean our Diagnostic session request is not correct or the module does not recognize the request.  See the page for Negative response codes.

0x01 – Default Session

0x02 – Programming Session 

0x03 – Extended Session

Service $11 “ECU Reset” (Allowed During Default Session) – The ECU rest service is used to reset or restart the ECU after a flash programming or used to return the ECU to the default session (the default session is the state that the vehicle or ECU normally runs in when the vehicle is in normal operation. Think of it like installing a fresh copy of an operating system of Windows  or Apple IOS on your home computer. The software is installed within the boot software, after the installation it’s required to perform a reboot for the system to run the application software which is the Windows or Apple IOS. ECU’s in the vehicle it works the same way, There is a boot software and an application. The application is the software that runs the Engine program if its an engine module. 

0x01 – Hard Rest

0x02 – Key Off On Rest

0x03 – Soft Reset

Service $14 “Clear Diagnostic Information” – This Service is used to clear DTC information from the ECU 0x000000 – Emission Related DTCs

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