Well, Yankee is correct and I’ll have to eat a little crow. I said before on here that you can fill it all the way up and not have a problem. I did this for years on my Cummins Ram. However, you CAN overfill the DEF tank and it will indeed throw a code. I just did this. By overfill, I mean fill it slowly and keep adding as the fluid level goes down the filler tube. I put in just over 2gal really easy and then it burped. At that point.....STOP!!!! I didn’t. I was able to squeeze in the last 0.5 gal or so. Got in the truck and drove 25min to go mt biking. Gauge didn’t move off 3/4. Started my drive home and gauge didn’t move. After stopping for food, now the 3rd startup cycle, truck throw a CEL. Stop by Autozone and code read it. P203E, DEF level sensor code. Drained out some DEF (which is a nasty procedure on the Ram) and gauge went to 100% on the next startup. CEL was still on, so stopped by the dealer to have them reset it. Tech said the new trucks and Jeeps have a problem with overfilling and throw the CEL. He’s seen it a bunch and hoping there will be a software update to stop the CEL. He said when it burps back on you, it’s full and stop filling.