That's OK, it took a while, but we finally figured it out. I've seen bulbs being burnt out trip a computer, but this is the first time I've seen one just missing and not required do it. That's the fun of getting a lift that we haven't serviced.

Sorry about the slip up. I should have referred to my own personal notes before I mentioned the Neutral light as a possibility. I have encountered more issues where a faulty Neutral indicator bulb was the culprit. Only had 1 case of where the Mast Locked bulb was burned out and I guess it didn't aggravate me enough for the memory to be bitter.
Oh well, mistakes should keep us humble, and hopefully do better.

Sure was.

My bad! I forgot to also mention that the Dash 14s that have the Komatsu Operator Presence Sensor system also feature a "Mast Locked Lamp" even if the truck DOES NOT have the hydraulics lockout featured in the configuration.
I know that the 2006 year models were only required to interrupt the transmission drive if seat is vacant, so the trucks built as 2006 models do not have the necessary locking solenoids for the mast functions installed, but they still have a "mast lock" bulb and if that bulb is burned out (or missing) it too will upset the KOPS controller when it does the bulb check at start up.
2007 year models were required to interrupt transmission drive AND mast functions if the seat is vacant.
So let me guess, the missing bulb was the one to the left of the engine MIL.

I figured that I should update for everyone. Well, we had two problems. The first is that someone at some point of time pulled the O2 sensor's heater fuse & never put one back in.

However, the 3 flashing issue was because someone also just straight up removed a light from the dash pod. The neutral light was still good. However, we could tell that at one point there was a light in one of the other holes that had no display due to the contacts having been rubbed clean. On a whim, we threw one in & it fixed it. I guess that person thought it was a spare.

Thanks, this gives us a place to start at least. I'll update after some troubleshooting.

From what you are stating, I will presume that this particular FG20-14 has an Operator's Presence Sensor (seat switch).
Here's what you are actually looking at.
The "10 flashes" of MIL means that the engine ECU has no fault codes stored (that would actually be 10 flashes repeated 4 times, which represents "4 zeroes").
The "3 flash" code you are seeing is being generated from the KOPS controller indicating a fault in the Komatsu Operator's Presence Sensor system.
A 3 flash code can mean a few things but I have found from experience that this is most often caused by a "burned out" Neutral indicator bulb. Because the KOPS system shares the engine MIL for its notification of a defect it can sometimes be confusing as to just what is going on.
First thing you need to do in this code is to confirm whether or not the Neutral indicator bulb on the monitor panel lights up when the transmission is in Neutral.
The KOPS controller does a "self check" of all circuits whenever the ignition is turned on and an open circuit on the Neutral light bulb is considered a defective condition by the KOPS controller.
The thing to remember on these "dual purpose MIL" set ups is that to display stored codes from the engine ECU, you must use the pedal tap dance (or the diagnostic reader tool) to request the data to be given via MIL.
The KOPS system, in contrast, automatically displays its flashing codes when the ignition is turned on.
And if that is not enough to cause confusion, consider that the MIL will light up and stay lit continuously if certain codes are registered by the engine ECU.
And if the engine ECU has the MIL lit, any codes being given out by the KOPS controller are not obvious, because while the KOPS controller is giving out its own notification signal, the MIL is already lit continuously and the KOPS codes are invisible, until you resolve the issue with the engine ECU codes.

ForkliftAm;
You should be seeing a four didget P code number with the pedal diagnostic procedure.