13 Peterbilt 379 ISL9 def light on guage flashing

Hey, I'm working on a 2013 Peterbilt 379 with Cummins isl9 in it. Def gauge is at 99% but led on the gauge is flashing. This has caused the truck to go into a derate, only codes present are derate codes. I suspected a fualty def level sensor so I replaced it...did nothing for me, I can physically confirm that the tank is full. I can move the sensor and watch live data and it reads correctly. it performs and completes forced Regen, monitored live data all falls within range. Checked continuity from level sensor to ECM -good, ECM to gauge - good. Really stuck on this one. Any idea where to look? hard to crack with no actives codes leading to the fualt. What else could cuase that led to flash?
Any help would be great, thanks.

 

I'm suspecting that's what happen with the driver, but it's a large company and I can't get any info on what happen.
I have proper software able to clear codes but derates will go inactive once no active codes are present and this is not happening.. no fualts active to repair, got me going on this one

 

Gonna give the flash a try, you wouldn't happen to have access to cm2350 isl9 update file? =D