2001 Freightliner FLD with a Detroit 60, have issues

The red light came on, engine shut down. I had a mechanic look at it, the TRS and SRS sensors were replaced znd it still won't start. I bought it used so they have rigged it to where the codes don't show. My question is what are all the possible things that could be wrong if the red light comes on? Keep in mind injectors are good, no leaks, and sensors are replaced. Any ideas I would appreciate. Could it be the ECM?

 

None of those are issues all was checked and fine oil, coolant and so on

 

Like I said I don't know what the codes are because I bought it used and they have it rigged to where you can't see codes, seems odd but I didn't realize it when I bought it

 

But to hit the switch to keep the truck from shutting down, don't you need to do that before the truck shuts down? This scan gauge link is this something that a driver can buy to use on their own and figure out what's wrong with the truck?