Does this truck have Operator Presence Sensing System (seat switch) that disables the transmission drive if you rise up off the seat?
If so, there is a NEUTRAL lamp on the dash that should light up when the truck is in Neutral, and that light will flash on and off if the seat is empty with the transmission in FOR or REV.
Confirm that the NEUTRAL lamp is working by shifting to Neutral with the engine running.
If the NEUTRAL lamp DOES NOT light up, the engine MIL will light up because Komatsu decided to use the same lamp to indicate faults, whether from the OPPS controller or from the Engine ECM.
My experience with trucks in this series (with OPPS, or KOPS) is to confirm whether or not the NEUTRAL indicator lamp works.
If the truck has OPPS and the Neutral lamp does not work, that is a defective condition and will turn on the MIL.
It can be as simple as replacing a burned out Neutral lamp bulb, or there may actually be a fault in the engine.