If you've read this site much you've heard of "DEF header".
The def header is a shortened term that describes the module that is bolted to the top of the def tank, and the module assembly (header) contains the following components: Def heater, temperature sensor, level sensor, and the most important part in this duscussion, the def quality sensor.
The ECU (or ACU) needs to know the quality of the def. In other words, it wants to see 32.5% water, and 67.5% urea. There is a slight variation that is allowed, but IIRC only about 0.5% either way for both chemicals. The DQS sends the information it reads from the def tank to the ACU which works in conjunction with the ECU, which all do just a few things: One, sends you (the operator) a set of data whether that be just that the def tank is frozen or if the level is low/high/whatever, or if the def quality is outside the acceptable range.
So what people have realized-rightfully so-is that the entire def system is just another set of complications to an already complex system of running the engine. And with that, they're absolutely right. You have not only the header assembly, tank, and lines, you also have a def injector--which is screwed directly into the exhaust pipe, subject to HEAT, plus you have wiring which in a tractor or skid loader is subject to debris. Sticks, animals, whatever--all easily damaged and thus can get kinda expensive to fix.
The EPA is the root of all of it, but the EPA doesn't care how their emissions limits are met, but rather that they are met (in a nutshell). So, it's currently the least intrusive and least expensive to use the systems that are currently mass produced.
But now we see the downsides of not being self-sufficient. If we are reliant on someone else for something and that someone else decides they're not gonna send it no more, we get duped-exactly what I have been saying for decades.
