Diagnostic Trouble Codes (DTCs)

What is a DTC?

A Diagnostic Trouble Code is a standardized identifier for a fault condition. Technicians use DTCs to understand which system reported a problem.

SAE J2012 format

Example: P0301

• P = Powertrain
• 0 = SAE standard
• 3 = Ignition system or misfire
• 01 = Cylinder 1

DTC categories

Prefix	Category
P	Powertrain
C	Chassis
B	Body
U	Network / Communication

DTC status byte

Bit 7  Bit 6  Bit 5  Bit 4  Bit 3  Bit 2  Bit 1  Bit 0
WIR    TNCMC  TFSLC  TNCSLC  CDTC  PDTC   TFTMC  TF

• TF: testFailed
• TFTMC: testFailedThisMonitoringCycle
• PDTC: pendingDTC
• CDTC: confirmedDTC
• TNCSLC: testNotCompletedSinceLastClear
• TFSLC: testFailedSinceLastClear
• TNCMC: testNotCompletedThisMonitoringCycle
• WIR: warningIndicatorRequested

Pending vs confirmed

• Pending: fault detected but not confirmed.
• Confirmed: fault verified across cycles.
• Permanent: stored by emission rules in some systems.

Snapshot and extended data

• Snapshot (freeze frame) captures data at the time of failure.
• Extended data provides additional state and counters.

pyudskit usage

from pyudskit import UDS

uds = UDS()
print(uds.read_dtcs())
print(uds.explain_dtc("P0301"))
print(uds.decode_dtc_status(0x2C))