The vehicle’s computer works in conjunction with the cruise control module to have automatic speed control. If the PCM (Powertrain Control Module) receives an input signal showing that the vehicle cannot control its own speed, a test of the cruise control system will be started.
DTC code P0573 OBDII is stored when the PCM detects an abnormal resistance level or voltage in the cruise control brake switch circuit during that test. Basically, the error specifies that the vehicle speed cannot be controlled, so the cruise control is disabled.