Hello guys, welcome back to our blog. In this article, I will share the top 100 advanced-level UDS interview questions with answers, and these questions are most commonly asked by Daimler Trucks, Mercedes Benz, Borgwarner, Volvo, Bosch, Magna, and other OEM or product based automotive companies.

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Advanced Level UDS Interview Questions

01. What is UDS, and how does it differ from KWP2000?

Answer: UDS (Unified Diagnostic Services) is defined in ISO 14229 and is a modern diagnostic protocol for vehicle ECUs. It supports multiple services, including session management, security access, and routine control. KWP2000 (ISO 14230) is an older standard, less flexible, and mostly supported over K-Line. UDS can work over CAN, FlexRay, and Ethernet (DoIP), and it supports advanced functions such as non-volatile memory control and DTC handling, making it more suitable for modern ECUs.

02. What are the different UDS diagnostic sessions and their purposes?

Answer:

Default Session (0x01): Standard diagnostics with limited services.
Programming Session (0x02): Allows memory reprogramming.
Extended Diagnostic Session (0x03): Enables more services for advanced diagnostics.
OEM-defined Sessions: Custom sessions for specific OEM needs.

Each session changes the available services and permissions.

03. What is the format of a UDS message?

Answer: UDS messages follow a request-response model:

Request: [SID][Parameters]
Response: [SID + 0x40][Response Data]Example: Request = 0x10 0x03 (Start Extended Session), Response = 0x50 0x03

04. Explain Negative Response Code (NRC) 0x78.

Answer: NRC 0x78 means “Response Pending.” It is used when the ECU needs more time to process a request. The ECU replies with 0x7F [Original SID] 0x78 and later sends a positive or final negative response.

05. What is the difference between 0x10 and 0x11 services?

Answer:

0x10: Diagnostic Session Control – used to switch ECU diagnostic sessions.
0x11: ECU Reset – used to reset the ECU. Reset types include hard, soft, and key-off resets.

06. How does the UDS protocol handle security?

Answer: Via 0x27 (Security Access). The tester requests a seed, and the ECU responds. The tester calculates a key using a secure algorithm and sends it back. If the key is valid, the ECU unlocks secured operations (e.g., programming).

07. What are DIDs, and which service is used to access them?

Answer: DIDs (Data Identifiers) are unique IDs assigned to data items in an ECU. Services:

0x22: ReadDataByIdentifier – to read DID values
0x2E: WriteDataByIdentifier – to write DID values

08. Explain the flow of ECU flashing via UDS.

Answer:

Enter Programming Session (0x10 0x02)
Perform Security Access (0x27)
Erase memory (RoutineControl 0x31)
Transfer data blocks using 0x36 (RequestDownload) and 0x37 (TransferExit)
Reset ECU using 0x11

09. How does UDS handle long messages over CAN?

Answer: Through ISO-TP (ISO 15765-2):

Single Frame: ≤ 7 bytes
First Frame: Starts multi-frame
Consecutive Frames: Continue transmission
Flow Control: Sent by tester to manage flow rate and pacing

10. What is service 0x19 used for?

Answer: ReadDTCInformation. It allows retrieval of Diagnostic Trouble Codes, DTC status, snapshot data, and extended data. Subfunctions allow filtering by status mask, severity, or DTC origin.

11. How do you differentiate between a functional and physical request in UDS?

Answer:

Functional: Sent to a group (e.g., all ECUs), uses functional addressing (0x7DF)
Physical: Directed to a specific ECU (e.g., 0x7E0 for engine ECU)

12. What is a RoutineControl (0x31) used for?

Answer: It executes ECU-defined routines.

Modes:

StartRoutine
StopRoutine

RequestRoutineResultsExamples: erase memory, checksum, or system calibration routines.

13. What happens if the tester does not send KeepAlive (TesterPresent) messages?

Answer: The ECU exits the diagnostic session (returns to default session) after a timeout period (usually ~5s), ending access to extended features.

14. What is the role of the Flow Control (FC) frame in UDS?

Answer: In ISO-TP, Flow Control frames are sent by the receiver to:

Indicate readiness
Set Block Size (number of consecutive frames allowed)
Set STmin (minimum delay between frames)

15. What are the key timeouts defined in UDS?Answer:

P2: Time for ECU to send a response (~50ms typical)
P2*: Time for response after NRC 0x78 (~5s typical)
S3: Session timeout (~5s), ECU returns to default session

16. What is the use of service 0x85?

Answer: ControlDTCSetting – enables/disables DTC recording. Useful during reprogramming or testing to avoid logging known test errors.

17. Can UDS support Ethernet or only CAN?

Answer: UDS can work over multiple physical layers:

CAN (ISO 15765)
Ethernet via DoIP (ISO 13400)
FlexRay (ISO 10681)
K-Line (legacy)

18. What is the difference between NRC 0x12 and 0x13?Answer:

0x12: Sub-function not supported – valid SID, but sub-function unsupported
0x13: Incorrect message length or format – structure doesn’t match expectation

19. What is a Seed-Key mechanism in UDS?

Answer: Security Access is implemented via Seed-Key. ECU gives a seed, the tester uses a crypto algorithm to compute a key, and the ECU compares it. If valid, secure access is granted.

20. How do you handle NRC 0x33 (Security Access Denied)?

Answer: It indicates an invalid key or too many failed attempts. After repeated failures, the ECU may lock out security access for a defined time or until a power cycle.

21. What is the use of TesterPresent service (0x3E)?

Answer: It keeps the diagnostic session alive by preventing the ECU from timing out. It sends a periodic message indicating the tester is still connected and active.

22. Explain the use of 0x34 and 0x36 services.

Answer:

0x34 (RequestDownload): Initiates a download operation for ECU programming.
0x36 (TransferData): Transfers firmware data in chunks after the download request.

23. What is TransferExit (0x37) used for?

Answer: It signals the end of a data transfer sequence. The ECU verifies the data integrity and responds with confirmation if successful.

24. How is memory erasure handled in UDS?

Answer: Via service 0x31 (RoutineControl) with a specific routine ID that maps to memory erasure. Used before flashing new software.

25. What happens if you send a message with an unsupported service ID?

Answer: The ECU responds with a negative response: 0x7F [Invalid SID] 0x11 (Service Not Supported).

26. How do you confirm the successful completion of a routine?

Answer: Use 0x31 with sub-function “RequestRoutineResults.” The response contains routine status and result values.

27. Explain the format and function of NRC 0x22.

Answer: NRC 0x22 means “Conditions Not Correct.” It is returned when a request is not permitted due to session, security, or environment constraints (e.g., engine running).

28. What is the use of the 0x2F service?

Answer: IOControlByIdentifier. It allows control over ECU I/Os like turning ON/OFF actuators, fans, or relays temporarily for testing purposes.

29. How is error handling managed in UDS communication?

Answer: Through Negative Response Codes (NRCs) with 0x7F prefix. Each NRC code explains why a request failed. The tester can retry, change the session, or send another command based on the error.

30. Can an ECU support custom UDS services?

Answer: Yes. OEMs can define proprietary services outside the standard range (e.g., 0xB0–0xBF). These are documented internally and may include custom features for development or validation.

31. What is service 0x14, and when is it used?

Answer: Service 0x14 (ClearDiagnosticInformation) is used to clear stored DTCs from the ECU’s memory. It takes the groupOfDTC parameter (e.g., 0xFFFFFF to clear all). A successful response confirms erasure.

32. Can UDS services be executed conditionally?

Answer: Yes, UDS services can be gated based on conditions like ignition status, session, vehicle speed, or security access. The ECU will respond with NRC 0x22 (Conditions Not Correct) if the condition isn’t met.

33. What is the difference between 0x22 (ReadDataByIdentifier) and 0x23 (ReadMemoryByAddress)

Answer: 0x22 reads predefined data identified by a DID. 0x23 reads memory content at a specific address, which is useful in development or debugging. 0x23 usually requires a higher security level.

34. What is the meaning of NRC 0x31 (Request Out of Range)?

Answer: It indicates that a request contains a parameter (like DID or address) that is not supported by the ECU or is beyond its permissible range.

35. How is UDS over DoIP different from UDS over CAN?

Answer: UDS over DoIP (ISO 13400) uses Ethernet for high-speed diagnostics and flashing. It supports IP-based addressing, faster transfer rates, and remote diagnostics. The basic UDS message structure remains unchanged.

36. What is the purpose of a Functional Address in diagnostics?

Answer: Functional addresses (like 0x7DF in CAN) broadcast messages to all ECUs. Used for services like emissions testing or global DTC reads, where a group response is expected.

37. What are the primary subfunctions of service 0x19 (ReadDTCInformation)?

Answer: Common subfunctions:

0x01: ReportNumberOfDTCByStatusMask
0x02: ReportDTCByStatusMask
0x04: ReportDTCBySeverityMask
0x06: ReportDTCSeverityInformation

Each gives a different filtered or detailed DTC output.

38. What is the ISO-TP Block Size, and how does it affect UDS communication?

Answer: Block Size defines the number of Consecutive Frames that the sender can transmit before requiring a new Flow Control frame. It optimizes flow and prevents buffer overflows on the receiver side.

39. How do you handle a situation when multiple ECUs respond to a functional request?

Answer: Each ECU replies with a physical response. The tester must distinguish between responses using CAN IDs. In most tools, responses are filtered and matched based on expected ECU identifiers.

40. What is the difference between 0x27 subfunctions 0x01 and 0x03?

Answer: 0x01 is for requesting the seed (requestSeed), and 0x02 is for sending the key (sendKey). Similarly, 0x03/0x04, 0x05/0x06, etc., represent further levels of security.

41. How can you test the UDS protocol without a real ECU?

Answer: Using tools like Vector CANoe with UDS simulation, SoftECU, or dSPACE simulators. These can emulate ECU behavior and validate test sequences and services.

42. What are ExtendedDataRecords in DTC reporting?

Answer: These are additional details attached to DTCs, like runtime since failure, ignition cycle counts, or sensor values during the fault. Accessed using 0x19 subfunction 0x06 or 0x0A.

43. What happens if the tester sends TransferData (0x36) without RequestDownload (0x34)?

Answer: The ECU will respond with NRC 0x24 (Request Sequence Error) since 0x36 must follow a successful 0x34 initiation. The order of operations is mandatory in UDS.

44. What is the significance of suppressPosRspMsgIndicationBit?

Answer: It’s the 8th bit of a service ID. If set (e.g., 0x3E | 0x80 = 0xBE), the ECU will suppress the positive response (silent mode). Negative responses will still be sent.

45. How does UDS ensure data integrity during flashing?

Answer: Through CRC checks or custom checksums validated after TransferExit (0x37). If validation fails, the ECU sends an error or refuses to activate the software.

46. How is UDS compliance validated in ECUs?

Answer: Through conformance testing using standardized test suites like ISO 14229 or OEM-specific test plans. Vector, ETAS, and dSPACE offer test automation frameworks.

47. Can UDS services be tunneled over Bluetooth or USB?

Answer: Yes, the UDS protocol can be encapsulated over alternative transports like USB or Bluetooth for diagnostic tools. However, the framing and ISO-TP handling may differ.

48. How can you handle ECU session timeouts programmatically?

Answer: By monitoring the S3 timer (~5s). If inactivity is detected, send periodic 0x3E TesterPresent messages or re-initiate the session before the timeout occurs.

49. What is the role of Addressing Type in UDS?

Answer: Determines how messages are routed:

Physical addressing (to one ECU)
Functional addressing (to a group)

Addressing type affects ECU response behavior and message filtering.

50. Explain the UDS service 0x28 (CommunicationControl).

Answer: It controls the ECU’s ability to send or receive messages. Useful to reduce network traffic during programming. Modes:

Enable Rx/Tx
Disable Rx
Disable Tx
Disable both

Often requires elevated session or security access.

51. What is the role of the Session Layer in UDS communication?

Answer: The Session Layer manages diagnostic sessions, controlling access levels (Default, Extended, Programming). It ensures security, timing, and availability of services based on session state.

52. Explain what happens during a Session Transition (e.g., from Default to Programming).

Answer: The ECU validates the request (service 0x10), may require security unlocking (0x27), and reconfigures itself internally (e.g., disable watchdogs, allow memory write) for programming.

53. What is a DTC status byte, and what does each bit signify?

Answer: The status byte shows the current state of a DTC. Bits indicate if the fault is active, stored, pending, confirmed, warning indicator requested, etc., as per ISO 14229-1.

54. How does the security access service prevent replay attacks?

Answer: Seeds are generated freshly and expire after a short time or after a number of failed attempts. Some ECUs add timestamps or random elements to make seeds unpredictable.

55. What are the advantages of using UDS over DoIP in modern vehicles?

Answer: Faster flash programming, remote diagnostics, simultaneous access to multiple ECUs, integration with telematics units, and scalability with complex ECUs in ADAS/autonomous vehicles.

56. What is NRC 0x35 (Invalid Key), and how to handle it?

Answer: Sent when the ECU rejects a security key. The tester should retry with a correct key, ensure seed/key algorithms are synchronized, or perform a new seed request.

57. Explain UDS Service 0x85 (Control DTC Setting).

Answer: 0x85 enables or disables the setting of new DTCs temporarily. Useful during vehicle calibration, testing, or flashing to prevent false faults.

58. What are some challenges in implementing the 0x2F (InputOutputControlByIdentifier) service?

Answer: Challenges include ensuring real-time control of actuators, timing constraints, authorization checks, and restoration of I/O status after tests are completed.

59. Why is the 0x34 (RequestDownload) service critical for vehicle flashing?

Answer: It initiates secure memory download by negotiating memory address, size, and data format parameters. It ensures that only authenticated sessions can modify critical ECU data.

60. What are Precondition Failures, and how are they handled?

Answer: Occurs when a service is requested but prerequisites (ignition, session, voltage, etc.) aren’t met. ECU responds with NRC 0x22. The tester must check and satisfy the conditions first.

61. How does UDS ensure safe rollback if flashing fails midway?

Answer: Some ECUs implement dual-bank flashing or keep a backup partition. If the main flash fails, the ECU reverts to the last known good software upon reset.

62. What is the purpose of UDS Service 0x3D (WriteMemoryByAddress)?

Answer: Used to write data to a specific memory address, often with higher privileges. Mostly used during development; rarely enabled in production for security.

63. Explain how 0x2E (WriteDataByIdentifier) handles volatile vs non-volatile data.

Answer: Volatile data (e.g., RAM parameters) gets updated immediately. Non-volatile data (e.g., EEPROM settings) may require commit steps or have delayed storage to prevent memory wear.

64. What is the impact of not sending a TesterPresent (0x3E) during long operations?

Answer: ECU will timeout the session (S3 Timer expiry) and may abort ongoing operations like flashing, leading to incomplete programming or faults.

65. Describe the end-of-line (EOL) diagnostics process.

Answer: Final vehicle checks at production line using UDS: verifying DTCs, programming VIN, coding/calibration, disabling certain manufacturing-only modes before shipment.

66. What is a RequestCorrectlyReceived-ResponsePending (0x78) NRC?

Answer: It tells the tester the ECU is processing but needs more time before sending a final response. The tester should wait and retry after a short interval.

67. How does an ECU determine which UDS services to unlock during Extended or Programming sessions?

Answer: ECU firmware maps services to access levels; upon session switch, internal flags determine what features/services become available.

68. What is the role of Parameter IDs (PIDs) in UDS vs OBD protocols?

Answer: PIDs in OBD (service 0x01, 0x09) are standard identifiers for emissions diagnostics. UDS uses DIDs (Data Identifiers) for broader, customizable vehicle/ECU data beyond emissions.

69. Why are some UDS services like 0x2F (I/O Control) or 0x86 (ResponseOnEvent) rarely implemented?

Answer: They add complexity, require real-time behavior, and pose security risks if mishandled. OEMs enable them only where absolutely needed.

70. Explain the use of 0x2C (DynamicallyDefineDataIdentifier).

Answer: Allows creation of a custom DID by combining several signals dynamically. Useful for aggregating data in test scenarios or customized readouts without modifying ECU firmware.

71. How does UDS handle timing for long-duration services like ECU flashing?

Answer: UDS uses Positive Response Suppression (e.g., with 0x78 NRC) and requires periodic TesterPresent messages to keep the session alive while the ECU processes lengthy operations.

72. What is the difference between synchronous and asynchronous UDS services?

Answer: Synchronous services give immediate response (e.g., 0x22 ReadDataByIdentifier), while asynchronous services (e.g., 0x31 RoutineControl) may return a pending response (0x78) before the final positive or negative response.

73. What is the maximum length of data that can be transferred in a single UDS message over CAN

Answer: A single CAN frame carries up to 8 bytes. For larger data, UDS uses ISO-TP (ISO 15765-2) to segment and reassemble messages, supporting up to 4095 bytes.

74. Explain the structure of a RoutineControl request (0x31).

Answer: It includes a service ID (0x31), sub-function (e.g., start/stop routine), routine ID (2 bytes), and optional routine parameters, depending on the routine implementation.

75. What is the effect of incorrectly configured session timeouts (e.g., S3 timer)?

Answer: Too short a timeout may disconnect the tester prematurely; too long may pose security risks. Optimal timeout ensures reliable session handling and secure diagnostics.

76. Why are certain services blocked in DefaultSession?

Answer: To protect critical ECU functions from being accessed or changed unintentionally. Services like 0x2E, 0x31, and 0x34 are reserved for Extended or Programming sessions with security access.

77. What are the most common NRC codes encountered during diagnostics?

Answer: 0x10 (General Reject), 0x11 (Service Not Supported), 0x12 (Sub-function Not Supported), 0x22 (Conditions Not Correct), 0x35 (Invalid Key), and 0x78 (Response Pending).

78. How does UDS manage concurrent access from multiple testers?

Answer: Most ECUs allow one active diagnostic session at a time. Some may implement priority-based or session-lock mechanisms to avoid conflicts or unauthorized access.

79. Explain the use of Functional addressing vs Physical addressing.

Answer: Functional addressing sends a broadcast message to all ECUs (e.g., emissions test), while physical addressing targets a specific ECU for configuration or control operations.

80. How can DoIP (Diagnostics over IP) affect traditional UDS communication?

Answer: DoIP enables faster and parallel diagnostics, especially for high-bandwidth tasks like flashing, but also requires secure network handling, routing, and session management.

81. How are large datasets read using 0x23 (ReadMemoryByAddress)?

Answer: Large memory reads are handled using ISO-TP segmentation and may require multiple read cycles, with careful address and block size management.

82. What is the role of a DCM (Diagnostic Communication Manager) in the AUTOSAR UDS stack?

Answer: DCM handles session management, request dispatching, service processing, and interaction with application-level components and memory for UDS services.

83. Why is it risky to implement unrestricted 0x3D (WriteMemoryByAddress)?

Answer: It can allow writing into sensitive ECU memory (e.g., bootloader or calibration), potentially bricking the device or exposing vulnerabilities if not secured properly.

84. What are the differences between session timing parameters P2 and P2*?

Answer: P2 is the normal response time after a request; P2* is the extended time used when ECU needs longer processing (used in conjunction with 0x78).

85. What is the typical use case of UDS Service 0x29 (CommunicationControl)?

Answer: To enable or disable communication (transmit/receive) of specific protocols (CAN, LIN, etc.) during tests or reprogramming to avoid bus overload or interference.

86. How do DIDs differ from OBD PIDs in implementation?

Answer: DIDs are OEM-defined and can represent any data, including non-emission-related info. PIDs are standardized (SAE J1979) for emissions monitoring and are limited in scope.

87. How does UDS implement access control across different diagnostic sessions?

Answer: Each session has predefined access rights. For example, ProgrammingSession allows flash operations, while ExtendedSession permits certain calibrations. SecurityAccess may also be layered.

88. What is the significance of sub-functions in services like 0x10, 0x11, 0x85?

Answer: Sub-functions refine the operation (e.g., selecting a session type, reset type, or DTC enable/disable), making services more versatile and context-dependent.

89. What happens if a tester sends a service that’s not supported in the current session?

Answer: ECU responds with NRC 0x7F followed by 0x78 or 0x22, depending on whether the service is not allowed due to current conditions or session type.

90. How is data consistency ensured during WriteDataByIdentifier (0x2E) operations?

Answer: ECU may use internal commit steps or checksum validation to ensure data is written correctly. Some systems delay writing until all parameters are validated.

91. What is the role of ISO-TP in UDS?

Answer: ISO-TP (ISO 15765-2) enables segmentation and reassembly of UDS messages over CAN, handling flow control, sequence numbers, and error recovery.

92. Explain the significance of suppressing positive responses using bit 7.

Answer: By setting bit 7 of the sub-function byte, the tester can request no positive response (for example, when broadcasting commands), reducing bus traffic.

93. How does the ECU handle excessive security access failures?

Answer: Most ECUs implement a lockout timer or permanently lock the diagnostic session until reset, to prevent brute-force attacks.

94. What challenges arise during UDS implementation over FlexRay or Ethernet?

Answer: Timing synchronization, packet loss recovery, and gateway routing complexities are increased compared to CAN-based systems.

95. Can an ECU respond with multiple NRCs for a single service?

Answer: No, only one NRC is allowed per service request, representing the highest-priority failure reason encountered.

96. What is the significance of diagnostic service 0x19 (ReadDTCInformation)?

Answer: It provides DTC-related information like status, snapshot data, and extended data, crucial for post-production diagnostics and field servicing.

97. How do flash sessions differ in terms of ECU resource usage?

Answer: Flash sessions typically disable normal ECU operations, watchdogs, and communication to allocate all resources to memory erase/write and ensure stability.

98. What are Freeze Frame and Extended Data Records in DTCs?

Answer: Freeze Frame captures the ECU state when a DTC is triggered. Extended Data Records store additional fault context for deep analysis.

99. What is the difference between routine control and direct I/O control services?

Answer: RoutineControl executes predefined internal routines (e.g., erase memory), while InputOutputControl directly influences actuators or sensors temporarily.

100. What are some best practices for secure UDS implementation?

Answer: Use strong seed/key algorithms, restrict critical services in production, validate all requests thoroughly, use session timeouts, and log access attempts for analysis.

This was about “Advanced Level UDS Interview Questions“. Thank you for reading.

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