Today’s cars are no longer just mechanical systems. They are complex networks of Electronic Control Units (ECUs). These units communicate with each other using standardized protocols. Among the most important of these protocols is the Unified Diagnostic Services (UDS).
In this article, we will discuss what UDS is. We will explain how it works. We will also explore why it is so important in car diagnostics, firmware updates, and ECU communications.
What Is Unified Diagnostic Services (UDS)?
Unified Diagnostic Services (UDS) is a standardized communication protocol used in automotive ECUs.
It enables:
- Vehicle diagnostics
- Firmware and software updates
- Fault code reading and clearing
- Parameter adjustments
- System testing and calibration
UDS defines how a diagnostic tester communicates with ECUs in a structured and reliable way. In simple terms we can say that, UDS is the “language” that diagnostic tools use to talk to vehicle electronics.
The Automotive Diagnostic System Architecture
A typical automotive diagnostic system consists of three main components:
1. Diagnostic Tester (Client)
This is the device used by technicians, which may be:
- A PC or laptop
- A tablet
- A handheld diagnostic device
It runs diagnostic software that sends and receives UDS messages.
2. Vehicle Communication Interface (VCI)
The VCI acts as a protocol converter. It:
- Connects to the tester via USB or RS-232
- Converts signals to CAN or other vehicle protocols
- Communicates with ECUs
3. OBD-II Interface
The system connects to the vehicle through the OBD-II port, also known as the J1962 connector. This is the physical access point for diagnostics.
Client–Server Model in UDS
UDS follows a client–server architecture:
- Client: The diagnostic tester (software + hardware)
- Server: The ECU’s diagnostic software
Each ECU contains a diagnostic server that handles requests from the client. Together, they enable standardized communication.
Unique Identifiers
- Every ECU has a unique server address
- Every diagnostic tester has a client address
These identifiers ensure that messages reach the correct destination.
ECU Network and Access
ECUs inside a vehicle communicate over CAN networks, which connect directly to the OBD-II port. This setup lets external diagnostic tools reach every ECU effortlessly.
The internal network layout does not matter. UDS (Unified Diagnostic Services) enables precise and individual addressing of each ECU for testing. It also allows for configuration.
Physical vs. Functional Addressing
UDS supports two addressing mechanisms:
1. Physical Addressing
- Communication is point-to-point
- Targets a specific ECU
- Used when the tester knows the ECU’s address
Example: Communicating directly with the engine control unit.
2. Functional Addressing
- Communication is broadcast-based
- Message is sent to multiple ECUs
- Used when the tester does not know which ECU provides a service
Example: Requesting emissions-related data from all relevant ECUs.
Request–Response Mechanism
All UDS communication follows a structured pattern:
- The client sends a service request
- The ECU processes it
- The server sends a response
There are two possible responses:
Positive Response
Indicates that the request was successfully processed.
Negative Response
Indicates that the request failed due to:
- Unsupported service
- Invalid format
- Incorrect conditions
- Security restrictions
Negative responses include a Negative Response Code (NRC) that explains the reason.
Service Identifiers (SIDs)
Each diagnostic request is identified by a Service Identifier (SID).
Key points:
- SIDs are 1-byte unsigned values
- There are 26 standardized UDS services
- Each SID represents a specific operation
These SIDs are grouped into six functional units.
UDS Functional Units
The 26 SIDs are organized into six categories:
1. Diagnostic and Communication Management (10 SIDs)
Manages sessions, security, and communication settings.
| SID (Hex) | Service Name | Description |
| 0x10 | DiagnosticSessionControl | Switches ECU between Default, Extended, Programming, etc. |
| 0x11 | ECUReset | Resets ECU (hard, soft, key-off, etc.) |
| 0x27 | SecurityAccess | Unlocks protected functions (seed/key) |
| 0x28 | CommunicationControl | Enables/disables CAN/LIN/FlexRay messages |
| 0x3E | TesterPresent | Keeps diagnostic session alive |
| 0x83 | AccessTimingParameter | Reads/writes timing parameters |
| 0x84 | SecuredDataTransmission | Encrypted communication |
| 0x85 | ControlDTCSetting | Enables/disables DTC storage |
| 0x86 | ResponseOnEvent | ECU sends data on trigger |
| 0x87 | LinkControl | Changes baud rate / communication speed |
2. Data Transmission (7 SIDs)
Handles real-time data exchange between client and ECU.
| SID (Hex) | Service Name | Description |
| 0x22 | ReadDataByIdentifier | Reads data via DID (VIN, RPM, temp, etc.) |
| 0x23 | ReadMemoryByAddress | Reads ECU memory |
| 0x24 | ReadScalingDataByIdentifier | Reads scaling info |
| 0x2A | ReadDataByPeriodicIdentifier | Periodic data reporting |
| 0x2C | DynamicallyDefineDataIdentifier | Creates custom DIDs |
| 0x2E | WriteDataByIdentifier | Writes configuration data |
| 0x3D | WriteMemoryByAddress | Writes directly to memory |
3. Stored Data Transmission (2 SIDs)
Used for reading and clearing Diagnostic Trouble Codes (DTCs).
| SID (Hex) | Service Name | Description |
| 0x19 | ReadDTCInformation | Reads stored faults and freeze frames |
| 0x14 | ClearDiagnosticInformation | Clears DTCs and related data |
4. Input/Output Control (1 SID)
Controls simple ECU outputs.
| SID (Hex) | Service Name | Description |
| 0x2F | InputOutputControlByIdentifier | Controls actuators (relays, motors, LEDs, etc.) |
5. Routine Control (1 SID)
Used for complex diagnostic routines and internal functions.
| SID (Hex) | Service Name | Description |
| 0x31 | RoutineControl | Start/Stop/RequestResult of routines |
Subfunctions
| Subfunction | Meaning |
| 0x01 | StartRoutine |
| 0x02 | StopRoutine |
| 0x03 | RequestRoutineResults |
6. Upload/Download (5 SIDs)
Supports ECU programming and firmware updates.
| SID (Hex) | Service Name | Description |
| 0x34 | RequestDownload | Requests permission to send data |
| 0x35 | RequestUpload | Requests permission to read ECU memory |
| 0x36 | TransferData | Sends/receives data blocks |
| 0x37 | RequestTransferExit | Ends data transfer |
| 0x38 | RequestFileTransfer | File-based transfers (optional) |
Diagnostic Sessions in UDS
UDS operates in different diagnostic sessions. Each session enables specific services and permissions.
Default Session
- Active when ECU is powered on
- Limited diagnostic access
- Used for basic operations like reading DTCs
Non-Default Sessions
These include:
- Programming Session
- Extended Diagnostic Session
- Safety System Diagnostic Session
- Manufacturer-Specific Session
- Supplier-Specific Session
Each session unlocks additional services depending on its purpose.
Supported SIDs by Session Type
Not all SIDs are available in every session.
- Default session supports only basic services
- Non-default sessions support all 26 SIDs
This ensures safety and prevents unauthorized operations during normal vehicle operation.
Diagnostic Session Transitions
ECUs manage transitions between sessions carefully:
- Power-on → Default session
- Default → Other session (e.g., Programming)
- Transition between non-default sessions
- Return to Default session
During transitions:
- Active routines may be stopped
- Security access may be reset
- Temporary configurations are cleared
This maintains system stability and safety.
Diagnostic Session Control (SID 0x10)
The Diagnostic Session Control service (0x10) is used to change sessions.
Request Format
The tester sends: [0x10] [Sub-function]
Example:
- 0x01 → Default session
- 0x02 → Programming session
- 0x03 → Extended session
Each sub-function corresponds to a specific session.
Positive Response for 0x10
If the request succeeds, the ECU responds with: SID + 0x40
Example:
- Request: 0x10
- Positive response: 0x50
This indicates successful session change.
Negative Responses for 0x10
If the request fails, the ECU sends: 7F [Original SID] [NRC]
Common NRCs include:
| Code | Meaning |
| 0x12 | Sub-function not supported |
| 0x13 | Incorrect message length/format |
| 0x22 | Conditions not correct |
“Conditions Not Correct” (0x22)
This often occurs when:
- Vehicle is moving
- Engine is running
- Safety requirements are not met
Many diagnostic operations require the vehicle to be stationary and in a safe state.
Why UDS Matters in Modern Vehicles
As we know that UDS or Unified Diagnostic Services, is a important tool in the vehicle world. It helps fix and manage vehicles in many ways.
- Vehicle servicing and maintenance: UDS lets mechanics check car parts quickly. For example, they can read error codes from the engine or brakes without taking everything apart.
- Over-the-air (OTA) updates: Cars can get software fixes wirelessly, like updating your phone. UDS makes sure these updates go smoothly to the car’s computer.
- ECU flashing and reprogramming: ECU is the car’s brain. UDS helps update its software, like changing settings to improve fuel use or add new features.
- Fault analysis: When something goes wrong, UDS finds the problem fast. It shows clear details, like “low battery in the sensor,” so repairs are easy.
- Performance tuning: UDS lets you tweak the car for better speed or handling. Tuners use it to adjust engine power safely.
- Safety diagnostics: It checks safety systems like airbags or anti-lock brakes. This keeps drivers safe by spotting issues early.
As cars get smarter with more internet links and software control, UDS is at the heart of it all. In the new world of connected and software-driven vehicles, UDS keeps everything running smooth and secure.
Conclusion
Unified Diagnostic Services, or UDS, is the main system behind how we check and fix modern cars. It operates like a boss-worker setup. The client, like your scan tool, asks questions. The server, like the car’s brain or ECU, provides answers.
UDS uses clear codes called service IDs to handle jobs neatly. It also switches into special “sessions” to control what you can do like reading data, fixing faults or updating software. This setup brings big wins:
- Safe chats between tools and car parts: Messages get checked to stop hacks or errors.
- Trustworthy checks: You get real info on problems, like why the engine light is on, without guesswork.
- Risk-free software updates: Flash new code to ECUs without bricking the car.
- Same rules for all brands: Works the same on a Toyota, BMW or Tata no learning new tricks for each.
For car engineers, testers, and service professionals, understanding UDS completely is imperative. Modern vehicles, ranging from electric vehicles to smart SUVs, rely on it for rapid solutions and upgrades. If you Skip it, you will be left behind in the rapidly moving world of autos.
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