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The radio-frequency (RF) or microwave-based catalyst state determination offers the opportunity to operate an automotive catalyst at its optimal point. This has already been proven for the oxidation state of TWCs, the soot loading state on DPFs/GPFs, and the ammonia storage state of 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 first results on an engine test bench with a serial-type zeolite-based SCR catalyst, using urea solution and the RF tool to detect the current ammonia loading in real time and to control directly the urea dosing system without any additional sensors. The original catalyst volume was reduced by 50 % to operate deliberately the SCR system under high space velocities and to challenge its function. Stationary conditions and operation points with continuously changing NOx emissions and space velocities were observed.