The error code P0731 OBD2 is set on vehicles equipped with automatic transmission. The reason for its setting has to do with failure when driving in 1st gear or when shifting to 6th gear.
To increase vehicle speed and maximum engine power output, vehicle computer-controlled transmission uses various transmission ratios.
The Engine Control Module (ECM) uses the information provided by transmission input and output sensors to compare speeds, and it verifies that the precise transmission ratio is present in the 1st gear. If the ECM determines that the transmission ratio is outside of the specified value, this DTC code is set.