SPN 4364 FMI 18?

[JMBT]

Well it sounds like you are on the right track! I had this code so often I threatened to buy a DEF refractometer, online about $50, but i haven't. Even buying DEF in the jug there is no way to know if it has reached it's shelf life of one year! Yeah, that's right it only has a shelf life of 1 year, then it goes bad. It wouldn't surprise me if the truck stops dump all their old DEF from the jugs into their bulk tank! I found the best way to keep that code at bay was to top off the DEF tank at every fuel stop, in an attempt to diluted any poor quality fluid I may have gotten. And always clean the nozzle off with a paper towel before filling your tank. Even the crystals that form on the nozzle can contaminate your system.