Session Management Services

DiagnosticSessionControl (0x10)

Switch the ECU between default, programming, and extended sessions.

Request Format

Byte	Field	Value	Description
0	SID	0x10	Service identifier
1	subFunction	0x01/0x02/0x03	Session type

Response Format (Positive)

Byte	Field	Value	Description
0	SID	0x50	Positive response
1	subFunction	echo	Echo of request
2–3	P2ServerMax	ms	Normal response timing
4–5	P2*ServerMax	ms	Extended response timing

Negative Responses

NRC	Meaning
0x12	subFunctionNotSupported
0x7E	subFunctionNotSupportedInActiveSession
0x22	conditionsNotCorrect

Subfunctions

SubFunction	Name
0x01	defaultSession
0x02	programmingSession
0x03	extendedDiagnosticSession

Session Availability

All sessions.

pyudskit Example

from pyudskit import UDS

uds = UDS()
print(uds.switch_session("extended")["uds_bytes"])  # 10 03

Session resets

Some ECUs reset timers and disable services after session switches. Use TesterPresent to keep the session alive.

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ECUReset (0x11)

Request a reset of the ECU.

Request Format

Byte	Field	Value	Description
0	SID	0x11	Service identifier
1	resetType	0x01/0x02/0x03	Hard/KeyOffOn/Soft

Response Format (Positive)

Byte	Field	Value	Description
0	SID	0x51	Positive response
1	resetType	echo	Echo of request

Negative Responses

NRC	Meaning
0x12	subFunctionNotSupported
0x22	conditionsNotCorrect

Subfunctions

SubFunction	Name
0x01	hardReset
0x02	keyOffOnReset
0x03	softReset

Session Availability

All sessions.

pyudskit Example

from pyudskit import UDS

uds = UDS()
print(uds.ecu_reset("hard")["uds_bytes"])  # 11 01

Side effects

ECU reset may drop the connection. Reconnect before sending further requests.

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SecurityAccess (0x27)

Unlocks protected services via seed-key exchange.

Request Format

Byte	Field	Value	Description
0	SID	0x27	Service identifier
1	subFunction	odd/even	Seed request or key send
2–N	data	seed/key	Optional key bytes

Response Format (Positive)

Byte	Field	Value	Description
0	SID	0x67	Positive response
1	subFunction	echo	Echo of request
2–N	seed	bytes	Seed for key calculation

Negative Responses

NRC	Meaning
0x35	invalidKey
0x36	exceedNumberOfAttempts
0x37	requiredTimeDelayNotExpired

Subfunctions

SubFunction	Name
0x01	requestSeedLevel1
0x02	sendKeyLevel1
0x03	requestSeedLevel2
0x04	sendKeyLevel2
0x05	requestSeedLevel3
0x06	sendKeyLevel3

Session Availability

Extended, Programming.

pyudskit Example

from pyudskit import UDS

uds = UDS()
print(uds.security_access_seed(level=1)["uds_bytes"])  # 27 01

OEM algorithm

Key calculation is OEM-specific. pyudskit does not ship OEM secrets.

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CommunicationControl (0x28)

Enable or disable ECU communication.

Request Format

Byte	Field	Value	Description
0	SID	0x28	Service identifier
1	subFunction	0x00–0x03	Rx/Tx control
2	commType	0x01	Application network

Response Format (Positive)

Byte	Field	Value	Description
0	SID	0x68	Positive response
1	subFunction	echo	Echo of request

Negative Responses

NRC	Meaning
0x12	subFunctionNotSupported
0x31	requestOutOfRange

Subfunctions

SubFunction	Name
0x00	enableRxAndTx
0x01	enableRxDisableTx
0x02	disableRxEnableTx
0x03	disableRxAndTx

Session Availability

Extended.

pyudskit Example

from pyudskit import UDS

uds = UDS()
print(uds.communication_control("disable")["uds_bytes"])  # 28 03 01

Loss of comms

Disabling Rx/Tx may block future requests until reset.

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Authentication (0x29)

ISO 14229-1:2020 authentication services.

Request Format

Byte	Field	Value	Description
0	SID	0x29	Service identifier
1	subFunction	0x01–0x03	Auth steps
2–N	data	bytes	Auth data

Response Format (Positive)

Byte	Field	Value	Description
0	SID	0x69	Positive response
1	subFunction	echo	Echo of request
2–N	data	bytes	Auth response

Negative Responses

NRC	Meaning
0x34	authenticationRequired
0x50–0x5D	certificate/auth errors

Subfunctions

SubFunction	Name
0x01	requestChallenge
0x02	verifyResponse
0x03	authenticationComplete

Session Availability

Extended, Programming.

pyudskit Example

from pyudskit import UDS

uds = UDS()
print(uds.encode("Authenticate with ECU"))

Certificate flow

Authentication requires ECU-specific credentials and certificates.

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TesterPresent (0x3E)

Keep-alive to avoid session timeout.

Request Format

Byte	Field	Value	Description
0	SID	0x3E	Service identifier
1	subFunction	0x00 or 0x80	0x80 suppresses positive response

Response Format (Positive)

Byte	Field	Value	Description
0	SID	0x7E	Positive response
1	subFunction	echo	Echo of request

Negative Responses

NRC	Meaning
0x11	serviceNotSupported

Subfunctions

SubFunction	Name
0x00	zeroSubFunction

Session Availability

All sessions.

pyudskit Example

from pyudskit import UDS

uds = UDS()
print(uds.tester_present()["uds_bytes"])  # 3E 80

Suppress positive response

Some gateways ignore suppressed responses. Disable suppression if needed.

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AccessTimingParameter (0x83)

Read or set ECU timing parameters.

Request Format

Byte	Field	Value	Description
0	SID	0x83	Service identifier
1	subFunction	0x01–0x03	Timing operation
2–N	data	bytes	Optional timing data

Response Format (Positive)

Byte	Field	Value	Description
0	SID	0xC3	Positive response
1	subFunction	echo	Echo of request
2–N	data	bytes	Timing data

Negative Responses

NRC	Meaning
0x31	requestOutOfRange
0x22	conditionsNotCorrect

Subfunctions

SubFunction	Name
0x01	readExtendedTimingParameterSet
0x02	setTimingParametersToDefault
0x03	readCurrentlyActiveTimingParameters

Session Availability

Extended.

pyudskit Example

from pyudskit import UDS

uds = UDS()
print(uds.encode("Read currently active timing parameters"))

Timing mismatch

Incorrect timing parameters may break communication.

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SecuredDataTransmission (0x84)

Transmit secured data blocks.

Request Format

Byte	Field	Value	Description
0	SID	0x84	Service identifier
1–N	data	bytes	Secured payload

Response Format (Positive)

Byte	Field	Value	Description
0	SID	0xC4	Positive response
1–N	data	bytes	Response data

Negative Responses

NRC	Meaning
0x38	secureDataTransmissionRequired
0x39	secureDataTransmissionNotAllowed

Session Availability

Extended, Programming.

pyudskit Example

from pyudskit import UDS

uds = UDS()
print(uds.encode("Send secured data block"))

Security policy

Requires ECU-specific security provisioning.

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ControlDTCSetting (0x85)

Enable or disable DTC setting.

Request Format

Byte	Field	Value	Description
0	SID	0x85	Service identifier
1	subFunction	0x01/0x02	On/Off

Response Format (Positive)

Byte	Field	Value	Description
0	SID	0xC5	Positive response
1	subFunction	echo	Echo of request

Negative Responses

NRC	Meaning
0x7E	subFunctionNotSupportedInActiveSession

Subfunctions

SubFunction	Name
0x01	on
0x02	off

Session Availability

Extended.

pyudskit Example

from pyudskit import UDS

uds = UDS()
print(uds.control_dtc_setting("off")["uds_bytes"])  # 85 02

Diagnostics impact

Turning off DTC setting can hide faults. Use with care.

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ResponseOnEvent (0x86)

Configure ECU to respond on specific events.

Request Format

Byte	Field	Value	Description
0	SID	0x86	Service identifier
1	subFunction	0x00–0x03	Event type
2–N	data	bytes	Event configuration

Response Format (Positive)

Byte	Field	Value	Description
0	SID	0xC6	Positive response
1	subFunction	echo	Echo of request

Negative Responses

NRC	Meaning
0x12	subFunctionNotSupported
0x31	requestOutOfRange

Subfunctions

SubFunction	Name
0x00	stopResponseOnEvent
0x01	onDTCStatusChange
0x02	onTimer
0x03	onChangeOfDataIdentifier

Session Availability

Extended.

pyudskit Example

from pyudskit import UDS

uds = UDS()
print(uds.encode("Configure response on DTC status change"))

Event load

Excessive event reporting can overload the bus.

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LinkControl (0x87)

Control link-layer parameters such as baud rate.

Request Format

Byte	Field	Value	Description
0	SID	0x87	Service identifier
1	subFunction	0x01/0x02	Verify/Transition
2	baudrateIdentifier	byte	New speed

Response Format (Positive)

Byte	Field	Value	Description
0	SID	0xC7	Positive response
1	subFunction	echo	Echo of request

Negative Responses

NRC	Meaning
0x31	requestOutOfRange
0x22	conditionsNotCorrect

Subfunctions

SubFunction	Name
0x01	verifyBaudrateTransition
0x02	transitionBaudrate

Session Availability

Extended.

pyudskit Example

from pyudskit import UDS

uds = UDS()
print(uds.encode("Verify baudrate transition to 500k"))

Loss of communication

Changing baud rate can drop the link. Ensure both sides change together.