i think i just had an anuresim reading all that... lol
just kidding :o)

you do seem to have an odd problem here but maybe it isnt so odd.
After reviewing the service information on this truck from what i can get it is a Tier1 system?
It seems Tier 1 had alot of issues with these kind of things
- wire breakage
- bad relays (or relay box)
- bad grounding (at the frame ground point)
- connectors (corrosion, terminals,wire breakage)
these are just a few of the main examples we experienced with the cat trucks using this same system.
Komatsu does show a relay box that contains the same relays as the cat system does. I'm just wondering if its the same relay box. I would start there.

I did read through the EC part of the manual and there are several systems that can trigger the 'failsafe' mode and whatever is happening here is triggering the 'failsafe' and shutting off the LP solenoid as a safety precaution. I found it odd that the EGO circuit (code 0031) can also trigger this action according to the chart in the manual.
This code 1241 eludes me though as it is not listed in their code list. Not sure what it pertains to UNLESS when you installed the other ECU it was for a completely different model and this code was included in THAT particular model. I find that unlikely though because generally All Tier 1 systems utilize the same structure in codes. Maybe it is one of those 'unlisted' codes only the factory or component mfg knows about? Hard to say.
Maybe this 1241 was something that got stored in memory and is lingering until you do a diagnostic reset? i assume you already performed this proceedure? The code is still persistant?
Might want to try unhooking the battery for an hour or so or even overnight to clear all memory. Then start up the truck and see what pops up then? If it starts bringing up random codes again i would look back at the relay box, mainly the EGI relays or even the box itself. We had several box's go bad internally due to the metal bars inside it breaking due to incorrect assembly, they were under a strain and cracked and broke internally because of this. The ultimate result was codes popping up and engine not wanting to start.


Oh yeah, if you do have the SST tool for this truck try testing the EGO sensor with it instead of ohming it out. Even if it does pass and there is a question if its really good or not i'd replace it anyway. Have seen issues with pigtails on some of these sensors.

Just a few things that came to mind while reading your post while eating a sammich and drinking a cool glass of tea ;o)