The value 0x7F can identify a negative-response message, while an NRC value of 0x7F means a service is unsupported in the active session. Its position in the message matters.
How to narrow it down
Keep the complete request and response instead of copying one byte from a log. Identify the requested service and the diagnostic session active at that time. This avoids describing every negative response as the same fault or mistaking protocol information for a stored vehicle DTC.
What to include with the fault report
Send the full message and the operation that produced it.
A hexadecimal number without its context is insufficient for a repair decision. The tool’s complete interpretation should be preserved.
Speak to AXION
For module programming and coding support, send your vehicle details and fault report. Workshop assessment is in Didcot, Oxfordshire. Suitable postal or remote work is confirmed before booking.
Technical background from py-uds. The referenced design explains the principle; the vehicle-specific procedure determines the repair.
Related reading — EEPROM page boundaries and data written to an unexpected address. Browse all vehicle-electronics diagnostic notes or return to the repair library.