What does it mean?
Today’s transmissions, whether they be manual, automated manual, adaptive dual clutch, or automatic, are more highly monitored and, in some cases, controlled by the engine control module and transmission control module (ECM and TCM). On some vehicles, the TCM may be integrated into the ECM. On automatic and automated-manual transmissions, the ECM and TCM work together to modulate engine output and transmission engagement, even tweaking gear engagement to be as smooth or powerful as desired.
Even on manual and adaptive dual clutch transmissions, the ECM monitors TCM and transmission readings to effect precisely-timed throttle blips and fine-tune engine output. Depending on transmission type and automaker, the TCM may use different sensors to monitor and control the transmission, such as speed sensors, pressure sensors, and temperature sensors.
Many of these transmissions also have gate select position (GSP) sensors, which may be mounted on the transmission, shift lever, or shift linkage. Position sensors tell the TCM which gear position the driver has commanded for at least a couple of reasons. On some transmissions, such as automated manual transmissions or automatic transmissions with manual mode, the GSP switch lets the TCM know whether or not the driver has selected automatic or manual function.
On manual transmissions, the GSP switch informs the TCM which gear the driver is selecting, and may function as a neutral safety switch, depending on system design. The TCM also uses the gate select position switch to monitor transmission operation. For example, if the driver selects manual mode and selects 3 rd gear on an automatic transmission, the TCM may monitor speed sensors to detect clutch and brake slippage.
If the ECM detects a problem with the GSP sensor circuit, it will store a diagnostic trouble code (DTC) in memory and illuminate the MIL (malfunction indicator lamp). DTC P0904 is defined as “Transmission Gate Select Position Circuit Malfunction,” and may be accompanied by other GSP sensor codes, such as P0905, P0906, P0907, or P0908.