u/Commercial-Farmer575

So i have a gmc terrain 2019 sle diesel 1.6, and at first my grandma borrowed it and saw the low def alert, didnt have any inside the truck so she went inside a convenience store inside a gas station, asked for def, and gave her a cleaning stp diesel for the fuel system, so she poured the whole thing, and i was afraid it was going to cristalize so i drained it, and then flushed with deionized water, and then drained it again and added new DEF, but then after a few months this warning of def quality poor came on, with a count down, but the count down usually restarted after a few minutes, and never actually limited the speed, and it usually just went away on its own, but then it didnt go away and the count down actually limited the speed, and it was fast, and then it got to 8 km/h, then i checked the codes with an obd machine, and it said that the nox sensor position 1 performance was not good, so i changed it, and now it’s still there, i did a reset, dpf forced regeneration, nox sesnsor reset, reductant fluid quality test, and none of them worked, im still in limp mode, and im thinking it might be my def now, but it doesnt really make sense, because i use the blue peak def, and i bought it not that long ago, but i still want to try that, and to clean my def injector, is there any other reason why it might be in limp mode? and is there an easy way to drain the DEF tank? because last time i used a small tube and just put it in the tube for the def, is there a plug or something for it that i dont know about?