Originally Posted by troverman

I'm curious how the truck "knows" the DEF is contaminated? I think the only way it concludes this is based upon the reporting of the NOx sensor. Basically the tank contains a heating circuit, the pump, and the 4 electrodes for measuring DEF level. The tank heater, pump heater, and line heaters are all controlled by two power circuits (MOSFETS, I think) in the glow plug module. They only heat based upon ambient temp. The pump cannot detect what it is pumping, and I don't believe the electrodes can do anything other than detect level by conductivity. That leaves only the dosing injector and the NOx sensor. The injector verifies its own operation and can report a fault code if it is not working properly...as can the NOx sensor. Therefore, if the PCM notes that their is DEF in the tank, the DEF injector is working, and the NOx is also working, but the NOx reading is not within acceptable range, the system concludes that the DEF is contaminated. DEF can get old and not perform as intended, or something other than DEF could be added (like water). The NOx is foolproof - the EPA wins if there is too much NOx coming out the tailpipe; the truck derates or shuts down.