UDS ISO 14229 Utility

UDS Diagnostic Message Decoder

Decode Unified Diagnostic Services messages from hexadecimal bytes. Identify UDS service IDs, request and positive-response services, negative responses, NRC values, diagnostic sessions, Data Identifiers, routine IDs, security access levels and common diagnostic sub-functions.

✓ Service ID ✓ Positive Response ✓ NRC ✓ DID ✓ Sub-Function

UDS

Diagnostic Message Decode

● Ready

Message Interpretation Auto mode detects 0x7F negative responses and standard SID + 0x40 positive responses.

Payload Display Payload meaning can be ECU, DID, routine or manufacturer specific.

UDS Diagnostic Hex Bytes Enter the reassembled UDS application bytes. Do not include ISO-TP PCI bytes such as Single Frame, First Frame or Consecutive Frame headers.

Important: UDS is the application-layer diagnostic protocol defined by ISO 14229. When UDS is transported over CAN using ISO-TP, first reassemble the ISO-TP payload and then enter the resulting UDS bytes here. This decoder does not treat ISO-TP PCI bytes as UDS service bytes.

UDS Diagnostic Decode Result Decoded

Decoded UDS Message —

Message Type —

Service ID —

Service —

Original Request SID —

Sub-Function —

Suppress Positive Response —

Data Identifier —

Routine Identifier —

Negative Response Code —

Payload Bytes —

Payload —

Total Message Bytes —

UDS Message Breakdown —

What Is a UDS Diagnostic Message?

Unified Diagnostic Services, commonly abbreviated UDS, is an automotive diagnostic protocol standardized by ISO 14229. It defines diagnostic services that allow a tester to communicate with electronic control units for tasks such as reading data, changing diagnostic sessions, performing routines, accessing security levels and transferring software.

A UDS message normally begins with a one-byte Service Identifier, or SID. The structure of the bytes following that SID depends on the selected service.

Common UDS Service IDs

SID	Service
0x10	Diagnostic Session Control
0x11	ECU Reset
0x14	Clear Diagnostic Information
0x19	Read DTC Information
0x22	Read Data By Identifier
0x23	Read Memory By Address
0x27	Security Access
0x28	Communication Control
0x2E	Write Data By Identifier
0x2F	Input Output Control By Identifier
0x31	Routine Control
0x34	Request Download
0x35	Request Upload
0x36	Transfer Data
0x37	Request Transfer Exit
0x3E	Tester Present
0x85	Control DTC Setting

UDS Positive Response

For many UDS services, a successful response uses a response SID equal to the request SID plus hexadecimal 0x40.

Request:
22 F1 90

Request SID:
0x22

Positive Response SID:
0x22 + 0x40
= 0x62

Response:
62 F1 90 ...

UDS Negative Response

A standard UDS negative response begins with 0x7F. The next byte identifies the requested service and the third byte contains the Negative Response Code, or NRC.

7F 22 31

7F:
Negative Response

22:
Original Requested Service
Read Data By Identifier

31:
Request Out Of Range

Common UDS Negative Response Codes

NRC	Meaning
0x10	General Reject
0x11	Service Not Supported
0x12	Sub-Function Not Supported
0x13	Incorrect Message Length Or Invalid Format
0x21	Busy Repeat Request
0x22	Conditions Not Correct
0x24	Request Sequence Error
0x31	Request Out Of Range
0x33	Security Access Denied
0x35	Invalid Key
0x36	Exceeded Number Of Attempts
0x37	Required Time Delay Not Expired
0x70	Upload / Download Not Accepted
0x71	Transfer Data Suspended
0x72	General Programming Failure
0x73	Wrong Block Sequence Counter
0x78	Request Correctly Received — Response Pending
0x7E	Sub-Function Not Supported In Active Session
0x7F	Service Not Supported In Active Session

Read Data By Identifier — SID 0x22

Read Data By Identifier requests one or more Data Identifiers, commonly called DIDs. A DID occupies two bytes.

22 F1 90

SID:
0x22

DID:
0xF190

DID 0xF190 is commonly associated with VIN data in standardized diagnostic implementations.

Diagnostic Session Control — SID 0x10

Diagnostic Session Control changes the active ECU diagnostic session. The byte following SID 0x10 contains the requested session type.

10 01
Default Session

10 02
Programming Session

10 03
Extended Diagnostic Session

Security Access — SID 0x27

Security Access is used where an ECU requires authorization before protected operations can be performed. Odd-numbered sub-functions commonly request a seed, while the corresponding even-numbered sub-function sends a calculated key.

27 01
Request Seed — Level Pair 1

27 02 ...
Send Key — Level Pair 1

The algorithm that converts a seed into a valid key is ECU-specific and is not inferred by this decoder.

Routine Control — SID 0x31

Routine Control can start, stop or request the result of an ECU-defined routine. A standard request contains a sub-function followed by a two-byte Routine Identifier.

31 01 FF 00

31:
Routine Control

01:
Start Routine

FF00:
Routine Identifier

Tester Present — SID 0x3E

Tester Present can be sent periodically to indicate that the diagnostic tester is still active and to help keep an applicable diagnostic session from timing out.

3E 00

Service:
Tester Present

Sub-function:
0x00

Suppress Positive Response Bit

For services that define a sub-function byte, bit 7 can be used as the Suppress Positive Response Message Indication Bit. The actual sub-function is obtained from the lower seven bits.

Sub-function byte:
0x83

Bit 7:
1 → Suppress positive response requested

Actual sub-function:
0x83 & 0x7F
= 0x03

UDS vs ISO-TP

UDS and ISO-TP operate at different layers. UDS defines diagnostic services and their application data. ISO-TP provides segmentation and transport when a diagnostic message is too large for a single CAN data field.

CAN / ISO-TP frame:
03 22 F1 90 00 00 00 00

ISO-TP PCI:
03

Reassembled UDS message:
22 F1 90

UDS SID:
22

Therefore this decoder expects 22 F1 90, not the ISO-TP PCI byte 03.

UDS Diagnostic Message Decoder FAQs

What does UDS stand for?

UDS means Unified Diagnostic Services.

Which standard defines UDS?

UDS diagnostic services are defined by the ISO 14229 family of standards.

What is a UDS SID?

SID means Service Identifier. It is normally the first byte of a UDS diagnostic request or positive response.

Why is a positive UDS response SID 0x40 higher?

For the common request/response service mapping, the positive-response SID is the request SID plus hexadecimal 0x40.

What does UDS 0x7F mean?

0x7F identifies a negative response message.

What does NRC 0x31 mean?

0x31 means Request Out Of Range.

What does NRC 0x78 mean?

0x78 indicates that the request was correctly received but the final response is not ready yet.

What is UDS service 0x22?

0x22 is Read Data By Identifier.

What is DID F190?

0xF190 is commonly used for the Vehicle Identification Number in standardized UDS implementations.

What is UDS service 0x27?

0x27 is Security Access, used for seed/key-based authorization mechanisms.

Can this tool calculate a UDS security key?

No. Seed-to-key algorithms are implementation-specific. This tool only decodes the UDS message structure.

Should ISO-TP PCI bytes be included?

No. Reassemble the ISO-TP message first and enter only the resulting UDS application bytes.

Decode UDS Diagnostic Hex Messages

Identify ISO 14229 services, request and response SIDs, diagnostic sub-functions, DIDs, routine identifiers, Security Access messages and negative response codes from raw UDS payload bytes.