Here is the principle of operation. Imagine a simple gear shift. Valves EV3 & EV4 move gear shift - left / right. EV1 & EV2 move forward / backward.
Two potentiometer inform ECU-where it is actually located gear shift. When you select first gear, EV3 moves the gear shift on the left. Potentiometer says to the ECU, that gear shift is against the left and ECU turn on EV1 valve. Gear shift going forward. When this happens, it is reported by other potentiometer to the ECU.
Second gear is engage with EV3 + EV2.
Fifth and rear are engage with (EV4 + EV1) for fifth and (EV4 + EV2) rear.
Third gear are engage only by EV1 for third and EV2 to fourth.
Very important is a gear shift to be located in the middle. (neither the left nor right)
As you see, your problem may be mechanical (the gear shift does not return alone in the middle), or one of the valves EV3 or EV4 not close well (leaking) and move the gear shift out from the middle.
You can remove the big nut at the top end of the robot, from where you will see what actually happens when switching. In this hole , you can put some (100ml) transmision oil .
PS: In Examiner, do not look for DTC codes. Just look for actual data and analize.