Vehicle diagnostics is the process of identifying, analyzing, and troubleshooting issues or faults within the electronic systems of a vehicle. It involves using specialized diagnostic tools, software, and equipment to access and interpret data from various ECUs in the vehicle.

What is Vehicle Diagnostics?

Vehicle Diagnostics

Vehicle Diagnostics is the exchange of Diagnostics data between ECUs and a tester tool. A Diagnostic protocol is required for communication of between them. Some examples of Diagnostic data are:

  • Monitoring of sensors and actuators
  • Detection of shortcuts and cut-offs
  • Detection of malfunctions
  • Management of error memory

Why Vehicle Diagnostics is important?

Vehicle diagnostics is crucial for several reasons:

  • Troubleshooting: Diagnostics help identify issues within a vehicle’s systems, allowing mechanics to pinpoint problems accurately. This saves time and reduces the guesswork involved in repairs.
  • Preventive Maintenance: Regular diagnostic checks can reveal potential problems before they escalate into major issues, thus preventing costly repairs and unexpected breakdowns.
  • Optimized Performance: Diagnostics can identify inefficiencies or malfunctions that affect a vehicle’s performance. By addressing these issues promptly, drivers can ensure their vehicles operate at peak performance levels.
  • Safety: Malfunctioning components or systems can compromise vehicle safety. Diagnostics help identify and address safety-related issues, ensuring the vehicle is safe to operate for both the driver and passengers.
  • Environmental Impact: Faulty emissions control systems can lead to increased emissions, contributing to pollution. Diagnostics can detect problems with these systems, helping reduce the environmental impact of vehicles.
  • Compliance: In many regions, vehicles must meet certain emissions and performance standards to comply with regulations. Diagnostics help ensure vehicles meet these standards, avoiding fines or penalties for non-compliance.

Different Vehicle Diagnostics Protocols

  • OBD-II: Onboard Diagnostic is a standardized diagnostic system (SAE J1979) in vehicles which monitors emissions, generates codes for faults and aids in troubleshooting.
  • UDS: Unified Diagnostic Services is a standard (ISO 14229) for vehicle diagnostics. It allows bidirectional communication between vehicle and diagnostic tool for advanced diagnostics.
  • DoIP: Diagnostics over Internet Protocol (ISO 13400) allows vehicle diagnostics over Ethernet/IP networks, enhancing speed and bandwidth for diagnostic communication.
  • KWP 2000: Keyword Protocol 2000 is a Automotive communication protocol (ISO 14230) for diagnostics and flashing ECUs. It supports both fast and slow baud rates.

Onboard Communication vs Diagnostics Communication

Onboard communication is the exchange of data between ECUs. All ECUs continuously send data messages over in-vehicle network (CAN, LIN, Ethernet etc). These messages are received by intended ECU and appropriate Diagnostic response is sent.

Whereas, Diagnostics communication is exchange of data between ECUs and a external tester tool as long as a diagnostic protocol is used. It is a request-response protocol. Unless there is a request made by tester tool, no response will be sent by an ECU. Examples of external test equipments (tester) includes:

  • OBD Scan tools
  • Workshop testers
  • Test rigs
  • Data Loggers
  • Calibration systems
  • Reprogramming systems
  • End-of-line testers in vehicle production
  • Diagnostic Software or in-vehicle network analysis tools (for example: CANoe, VehicleSpy)

Types of Vehicle Diagnostics response from ECU

There are three different types of Diagnostic responses from ECU. These Diagnostic responses from ECU are managed by CAN-TP or ISO 15765-2 standard.

Positive response

ECU receives the request from tester, acknowledges it and respond with positive response.

Negative response

ECU receives the request from tester but respond with a negative response due to reasons like:

  • Security level not sufficient
  • Requested diagnostic service not supported
  • Out of range parameter
  • Temporarily unable to perform

No response

ECU might not respond at all due to below reasons:

  • Malfunction in connectionLow BatteryDisconnectionShort cutWrong termination in the in-vehicle networkDefects in the ECU hardwareNon-matching Diagnostic protocol
  • Wrong communication parameters, wrong address, wrong data rate etc.

Service Identifiers (SID)

Every Diagnostic protocol consists of Diagnostic services (Example: Read Data by Identifier) which defines the type of service requested by the tester. There are 256 different values of SIDs ($00 to $FF). SID is 1 byte (8 bits).

2^8 = 256 different SIDs.

Response Service Identifiers (Resp SID)

These are the response SID sent by the ECU to the tester in response to a service request. They are also 1 byte long.

Response SID = Request SID + $40

Example: for a service $22 (Read Data by Identifier), response from ECU will be $22+$40=$62 (hex).

No Response Service Identifier

For a no response from ECU, a fixed value $7F is used.

A no response frame in CAN would look like this:

First Data ByteSecond Data ByteThird Data Byte
No Response SIDSID (Service Identifier)NRC (Negative Response Code)
No Response Diagnostic frame in CAN

Some examples of NRC according to UDS (ISO14229-1):

$10General Reject
$11Service not supported
$13Incorrect message length or invalid format
$21Repeat request
$31Request out of range
$78Request correctly received, response pending
$84Engine Not running
$85Temperature too high
$88Vehicle speed too high
$8FBrake pedal not pressed
$90Shift stick not in park
Examples of NRC (Negative Response Code)

Key aspects of Vehicle Diagnostics

  1. Fault Identification: Vehicle diagnostics aim to identify faults or malfunctions in the vehicle’s electronic systems, such as engine management, transmission, brakes, suspension, airbag systems, and more. These faults can be indicated by warning lights on the dashboard or by abnormal vehicle behavior.
  2. Diagnostic Tools: Automotive technicians use diagnostic tools and equipment to communicate with the ECUs and retrieve information about the vehicle’s condition. These tools can range from handheld scanners to more advanced diagnostic systems connected to a computer.
  3. Diagnostic Trouble Codes (DTCs): When a fault occurs in a vehicle’s system, the corresponding ECU generates Diagnostic Trouble Codes (DTCs). These codes provide specific information about the nature and location of the problem, aiding technicians in pinpointing the issue.
  4. Data Reading and Analysis: Diagnostic tools read and interpret data from various sensors, actuators, and control modules in the vehicle. This data includes parameters such as engine speed, coolant temperature, oxygen sensor readings, and more. By analyzing this data, technicians can identify patterns and anomalies that may indicate problems.
  5. On-Board Diagnostics (OBD): Most modern vehicles are equipped with On-Board Diagnostics (OBD) systems, such as OBD-II, which monitor the vehicle’s systems and emissions. OBD systems store DTCs and provide access to real-time data for diagnostics.
  6. System Calibration: In some cases, vehicle diagnostics may involve calibrating or reprogramming certain ECUs to resolve issues or improve performance. This process requires specialized calibration tools and software.
  7. Troubleshooting and Repair: Once the problem is identified through the diagnostic process, technicians can proceed with the necessary repairs or component replacements to rectify the issues.


Vehicle diagnostics are an essential part of automotive repair and maintenance, allowing technicians to quickly and accurately identify the source of problems and provide appropriate solutions. Diagnostic technology continues to advance, enabling faster data retrieval, enhanced analysis capabilities, and compatibility with various vehicle makes and models. Well-executed vehicle diagnostics help ensure safe and efficient vehicle operation, optimal performance, and compliance with environmental and safety regulations.