Could be an electrical connation, solenoid or sensor. Does it flash F then 4 on the dash? Easy first checks are to make sure the barrel connectors below are connected properly. They twist to secure. Located above the auto. Theres 2 of them. Also check the wires in the harness havent broken or snagged on something to cause a short to earth. Then do a resistance check on the sensors and solenoids. Disconnect the barrel connectors and use the connectors connected to the auto to measure the resistance of the sensors and solenoids. Resistance should be within certain limits. If not then this could point to a problem like a failed solenoid. Also replacing the auto oil can sometimes fix problems on this auto. The filler plug IS NOT the bolt on the side, which if taken off stops reverse gear from working as part of the mechanism falls off inside (take auto apart to put back). Theres a specific method to check the level of the auto fluid, which must be followed. Youd be betterer off buying the proper LR approved LRN402 oil too if you choose to change the auto oil, part no STC50531 for 1L
Barrel connectors x2
How to secure them when the fekers come apart
Jatco auto level plug and filler