UDS-Unified diagnostic services

UDS-Unified diagnostic services


  • All Modules continuously monitoring themselves and reporting their status.
  • Detecting a failure in complex machine would be a tedious task.
  • Two things are supposed to happen after detects the failure.
    • Malfunction Indication Light
    • Record the Memory.
  • Diagnostic equipment allows you to diagnose and fix the problem with the vehicle.
  • Diagnostic Tools are used to read data (DTC’s) from the EEPROM to analyze the cause of failure.
  • Equipment’s requires basically a communication medium and a communication protocol.
  • It makes possible to connect a computer (client)or diagnostics tool, which is referred to as tester, to the bus system of the vehicle.
  • The messages defined in UDS can be sent to the controllers which must provide the predetermined UDS services.
  • This makes it possible to interrogate the fault memory of the individual control units or to update them with a new firmware.


  • UDS derived from KWP2000
  • UDS communication happens over CAN
  • UDS – Specified in ISO 42219
  • Unified means that it is an international and not a company-specific standard

Usage of UDS

  • Debug the defective ECU in a vehicle where the ECU is not accessible for repair even at Garage
  • Flash the new SW
  • Update the EEPROM parameters
  • Reading and writing the ECU data (SW version, HW version, Address data, adaption data etc.,)

Basic Principles

  • The request / response message communication principle
  • Diagnostic services have a common message format.
  • Each service defines a Request Message, a Positive Response Message, and a Negative Response Message.
  • Positive Messages:
    • The Positive Response Message has an echo of the service ID with plus 40
    • The service-defined response parameters.
  • Negative Messages:
    • It has the Negative Response Service ID (0x7F) as first byte,
    • An echo of the original Service ID as second byte, and
    • A Response Code as third byte.

UDS message Format

  • Request Message format



  • Positive Response Message Format



  • Negative Response Message Format



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