What does it mean?
The automatic transmission does all the same things you would do to change gears in a manual transmission – modulating clutch application, gear position, throttle position, etc. When the automatic transmission was invented, it did all these things using an ingenious arrangement of spool valves, operated purely by variances in hydraulic pressure.
In the modern automatic transmission, most of the spool valves have been replaced by solenoid valves, controlled electronically by the transmission control module (TCM), which may or may not be a separate module from the ECM (engine control module). When the driver selects “D” on an automatic transmission, the TCM shifts through specific gear ratios by activating or deactivating of a number of shift solenoids which, in turn, control brakes, clutches, and bands in the automatic transmission.
The result, optimally, is smooth acceleration and shifting of gears from 1 st , 2 nd , 3 rd , and so on. During operation, the TCM checks the operation of the solenoid valves, consisting of a resistance check and speed sensor verification. If one of these checks fails, such as encountering an open circuit on Shift Solenoid F (SSF), the TCM disables that control circuit and sets DTC P0999 – Shift Solenoid F Control Circuit High in ECM memory.