There are 3 filters in the DEF system.  One is a paper element kinda like an oil filter element on a 4 wheeler.  The other two are actually strainers, in a 'housing' on the DEF module.  The first screen will be right there, when you take the housing apart, the other is up in the housing.  Gotta turn your head upside down to see it.  Takes a small pair of needle nose pliers to get it out.  That is the 'filter' that kept stopping up on my machine.  Now if you restart three times with the code up you will derate to idle and have to get a laptop hooked up to remove the derate.