Diesel Emission Fluid. Injected into the exhaust flow to convert the Nitrous Oxide into Nitrogen and water. It’s an emission thing. Basically it’s salt water. You can buy 2.5 gallon bottles at Wallymart/Car parts store or you can buy bulk at the truck stop. If your tank is on the driver side. Mine is on the passenger side. Mind boggling. Later coaches appear to have this problem fixed. Bit of a pain to get it in there. Now, as those that live in winter country know, salt is corrosive. So hose the whole thing down when you’re done filling. And, you may notice some serious corrosion on the metal bracket that holds the plastic tank. This should have been galvanized or at least powder coated. But I guess they didn’t. You don’t want this to rot through. So I got the wire brush and a can of rubberized under coating out.

There, that’s better:

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The second issue with the system can be a little more annoying. The quantity indicator is basically a float that rides up and down in the tank. Crystallization can make the float get stuck. Now your quantity indication is off. And if the engine THINKS it’s low on DEF it WILL shut you down. EPA and all that. Here is the “head” that’s mounted on top of the tank.

So I’ve had it happen that it thought it was low and threw an error. Filled it up and sloshing around the float got loose, took quite a few miles, and a few start/stops later the error reset itself.

So now I keep the tank at half or more…. There is a maintenance procedure that says to remove the head (after dropping the tank) and cleaning it all up in there. Hope to not have to do that.