State of the art for NOx reduction for low- and heavy-duty diesel engines is the ammonia SCR technique. Today’s SCR control approaches are model-based, relying on NOx sensors and the ammonia-providing urea dosing module. Based on model uncertainties, low ammonia storage levels on the catalyst surface are aimed to avoid ammonia slip. This leads to the disadvantage of catalyst oversizing or that the maximum catalytic potential is not exploited. The radio-frequency (RF) or microwave based catalyst state determination offers the ability to operate automotive catalysts at its optimal point. By using the catalyst canning as a resonator, knowledge about the electric properties of the catalyst can be derived from its resonance parameters. Their direct correlation to the catalyst state has already been proven for the oxidation state of a TWC, the soot loading on a DPF/GPF and the ammonia storage on vanadium and zeolite based SCR catalysts. However, the latter has only been demonstrated in laboratory scale with synthetic exhaust using gaseous ammonia. This work presents results on an engine test bench with a commercial zeolite based SCR catalyst, using urea solution and the RF state diagnosis to detect the current ammonia loading in real time and to directly control the urea dosing. Stationary conditions and operation points with continuous changing NOx emissions and space velocity were observed. In all performed measurements, high NOx conversion was archived and the ammonia loading could be detected. Possible influences of the urea dosing strategy to the measured ammonia storage degree were investigated. The RF signal was successfully used for closed-loop control of the urea dosing as two-point control with and without hysteresis. The performed experiments demonstrate that direct operation on a specific ammonia loading can both ensure maximum NOx conversion and avoid ammonia slip. By this RF control strategy, a catalyst volume reduction seems possible.