Diagnostic Sessions

A diagnostic session controls which services are available and what security is required.

Why sessions exist

ECUs restrict dangerous operations (programming, IO control) to special sessions. This prevents accidental or unauthorized changes.

Standard sessions

Session	SID	Purpose	Security Required
Default (0x01)	10 01	Basic diagnostics, read DTCs	No
Programming (0x02)	10 02	ECU flashing	Yes (SecurityAccess)
Extended (0x03)	10 03	Advanced diagnostics, I/O control	Sometimes

Timeouts

UDS sessions time out if no traffic is received. The standard keep-alive is S3 = 5000 ms. Use TesterPresent (0x3E) to keep the session alive.

Session transitions

pyudskit example

from pyudskit import UDS

uds = UDS()
uds.switch_session("extended")
print(uds.ecu_state)

Session support

If you receive NRC 0x7F or 0x7E, switch to the correct session before retrying.