What does it mean?
OBD II fault code P0996 is a generic code that is defined as “Transmission fluid pressure (TFP) sensor / switch “F” – circuit intermittent, and is set when the PCM (Powertrain Control Module) detects an intermittent failure in the control and/or signal circuit(s) of the transmission fluid pressure sensor/switch that is labelled “F”.
Note that on some applications there may be more than one transmission fluid pressure sensor/switch, which makes it important to refer to the manual for the affected application to determine exactly which transmission fluid pressure sensor/switch is labelled “F” on that application. WARNING: Failure to identify the affected sensor/switch correctly will lead to a misdiagnosis, and possibly the unnecessary replacement of parts and components.
All fully automatic transmissions use pressurized transmission fluid to control and manage gearshifts, regardless of whether the fluid is pressurized by an engine driven pump or by an external electrically operated pump. Moreover, the ultimate working pressure of the transmission fluid must fall within a narrow, predefined range, although this range varies depending on the application, as well as the design of the transmission.
In terms of operation, a modern automatic transmission uses electrically operated solenoids to channel pressurized fluid out of, and into a series of passages when shifting gears. While the pressurized fluid does not do the actual shifting of gears, it serves as the means by which various rotating (but also, non-rotating) parts and components are moved from one position to another in order to accomplish a gearshift.
For instance, when the TCM (Transmission Control Module) /PCM commands a gearshift, various solenoids in the transmission redirect pressurized fluid to first disengage a particular clutch pack before a set of planetary gears is reconfigured, much as different gears are selected/deselected in a manual transmission.
Once the planetary gear set is reconfigured to produce a different “gear ratio”, pressurized fluid causes another clutch pack to engage, which serves to keep the planetary gears in their current configuration. In addition, pressurized fluid may also (concurrently) be directed to the torque converter lock-up clutch to either engage or partially disengage both to make the gearshift smoother, and to protect vulnerable transmission parts against the application of excessive torque.
Thus, to monitor the pressure of the transmission effectively, most modern transmissions use one or more pressure sensors whose resistance changes in direct proportion to any changes in pressure.
In practical terms, a typical transmission fluid pressure sensor is supplied with a 5-volt reference voltage; as the pressure of the transmission fluid changes, the resistance of the sensor changes as well, which allows more, or less, current to be passed back to the PCM/TCM via a dedicated signal circuit. The changing signal voltage is then interpreted by the PCM/TCM as changes in pressure.
While the above is an over simplification of the processes that occur within a modern automatic transmission, it should nevertheless be obvious that the PCM/TCM requires accurate input data regarding the actual working pressure of the transmission fluid at all times.
If this input data is lacking or interrupted, the PCM/TCM cannot control the transmission effectively; therefore, if the data stream from the transmission fluid sensor(s) is interrupted for a period of time that exceeds a limit set by the manufacturer, code P0996 will be set, and a warning light may also be illuminated as a result.
Note that on some applications, the PCM/TCM may also initiate a failsafe or limp mode both as a safety measure, and to protect the transmission until the fault is corrected.