Diagnostic session (DiagnosticSessionControl 0x10)

What is a diagnostic session?

A diagnostic session is the operating mode of a UDS server, and DiagnosticSessionControl (0x10) is the service that selects it. Every ECU starts in the default session after power-up. The tester switches to another session to reach services that are not available by default.

A normal exchange looks like this:

Direction	Bytes	Meaning
Request	10 03	enter extendedDiagnosticSession
Positive response	50 03 00 32 01 F4	P2server_max 50 ms, P2*server_max 5000 ms
Negative response	7F 10 22	conditionsNotCorrect, e.g. vehicle speed not zero

Other typical negative responses are 7F 10 12 (subFunctionNotSupported), 7F 10 13 (incorrectMessageLengthOrInvalidFormat) and 7F 10 7E (subFunctionNotSupportedInActiveSession, for example programming session requested from default session when the ECU requires the extended session first). With the suppressPosRspMsgIndicationBit set, 10 83, the ECU switches without answering.

Where is it defined?

ISO 14229-1:2020 defines DiagnosticSessionControl (0x10) in the Diagnostic and communication management functional unit, together with ECUReset, TesterPresent and CommunicationControl. The standard lists the session types, the response parameter record and the rule that a session change resets session-dependent states, such as an unlocked security level. The transport underneath on CAN is ISO 15765-2.

What it means in practice

The session is the first gate in a diagnostic design. Which services and data identifiers answer in which session is defined by the OEM diagnostic specification, usually in an ODX or CDD file. In validation, engineers check:

• every service is available only in the sessions the specification lists, and answers 7F xx 7F (serviceNotSupportedInActiveSession) elsewhere;
• allowed and forbidden session transitions, for example programming only from extended;
• wrong length (10 alone or 10 03 00) gives NRC 0x13;
• the S3 timeout returns the ECU to default session and drops every state that belongs to the old session;
• the timing values in the response match the specification.

We regularly see services reachable in the default session that the specification places in the extended session. That is a configuration error, and it is easy to miss without a full sweep per session.

How AutoST tests it

AutoST enumeration discovers the sessions an ECU accepts, including extended and programming, and then sweeps every service ID from 0x00 to 0xFE in each session, classifying the answer. The result is a session-by-service matrix that you can compare against your diagnostic specification. The programming session is tested last and can be skipped, because some ECUs get stuck in it.