A multi-functional apparatus and method for diagnosing issues in an engine system with a SCR exhaust gas processing system and controlling reductant delivery to the SCR system. The apparatus includes a Venturi exhaust passage device with a reductant injector positioned on a transition from a low pressure passage to a downstream high pressure passage. Temperature and pressures in the Venturi exhaust passage device are measured, and an exhaust flow rate value is calculated. The exhaust flow rate value can be used in controlling reductant delivery, and with a downstream lambda sensing value, the exhaust flow rate value, and a temperature sensing value, a variety of issues in the engine system and the SCR system can be detected. No engine operating parameter is required in the SCR controls and diagnostic algorithms, and less reductant deposit is introduced due to the elimination of stagnant area in the exhaust passage device.