System and method for monitoring reductant quality

In a vehicle comprising an internal combustion engine and an emission control system comprising an SCR catalyst, a reductant storage device separate from a fuel storage device, and a reductant delivery system, a method for monitoring reductant quality in the emission control system, the method comprising:

• sensing an operating condition of the emission control system;
• detecting a deviation from said operating condition;
• determining whether the deviation occurred after a refill event of the reductant storage device separate from the fuel storage device, said refill event identified based on factors comprising at least one of an angle of inclination of the reductant storage device or a road condition;
• and
• if the deviation occurred after the refill event, then determining that a change in reductant quality due to reductant dilution has occurred within a time by addition of a reductant-diluting substance to the reductant storage device separate from the fuel storage device, the reductant reducing NOx in the SCR catalyst.