UDS service 0x85, known as Control DTC Setting, is a function used in the UDS (Unified Diagnostic Services) automotive diagnostic protocol that allows enabling or disabling the storage of DTC fault codes inside an ECU temporarily. This service is widely used during actuator tests, calibration procedures, module programming, and advanced diagnostic operations because these processes may generate temporary faults that should not be stored in the vehicle memory. By using commands such as 0x85 01 to enable fault storage and 0x85 02 to disable it, the scan tool can control how the ECU handles fault recording. When the ECU accepts the request, it responds with a positive frame 0xC5, which corresponds to service 0x85 + 0x40 according to the UDS standard. In many vehicles, this service requires first entering an extended diagnostic session (0x10 03) or performing a security procedure through Security Access (0x27). During automotive module analysis and ECU emulation practices, observing these communication frames is essential to understand how scan tools interact with ECUs and how diagnostic functions are internally managed within the vehicle.
