A method for diagnosing reductant delivery performance of a reductant delivery system is provided. The method provides an outlet pressure sensor at a reductant outlet of the reductant delivery system and a virtual pressure sensor in an exhaust stream where reductant is injected into the stream. The method comprises calculating a downstream reductant flow rate using the outlet pressure sensor and the virtual pressure sensor; and calculating an upstream reductant flow rate within the reductant delivery system using at least one of a temperature sensor, a pressure sensor and a control valve. The calculated downstream and upstream reductant flow rates are compared, and diagnostics is conducted based the comparison to find the malfunctions in the reductant delivery system. An intrusive self-consistent diagnostics method allows for detection of malfunctions in the outlet pressure sensor and the virtual pressure sensor can be determined.