I hate to hear your having this problem I fought it for a year or longer. The gauge is built into the tank so there is no replacing just the gauge. I removed the tank and filled with distilled water to get it to work once ,but even then it was picky ,if I filled it up the code would return. I could drain some back out without removing the tank and it would work correctly. When I got the 500 mile message again last month I drained the def removed the tank and soaked in distilled water again for a day to no avail. I finally had to just man up and have the tank replaced. Pulled into the dealer with 30 miles left to go on the restart message. I hate to be the bearer of bad news but it was $2k and some change. I posted the procedure I used to clean mine on this thread when I started it about a year ago you could try it and maybe it will work it’s worth a shot after all $8 worth of distilled water beats $2k any day. Draining and removing the tank isn’t all that difficult. I think the video someone else posted shows how to drain it really well. Removing the tank I removed the cross bar then the skid plate on the bottom of the tank then unplugged all the wires and the vent line then the two bolts that hold the tank straps on and then it will come out or fall on you if your not careful so make sure you drain it first. The fill line seems to have a flap inside to act as sort of a check valve so if you do remove it when you go to put the distilled water in make sure the funnel is small enough and long enough to push it in. Best of luck to you on what ever you decide to do
Sent from my iPhone using Tapatalk