This paper presents a fuzzy logic controller based ammonia detection algorithm to predict and capture ammonia slip for commercial vehicles. The current vehicle emission standard requires a well-designed selective catalytic reduction (SCR) system which must deliver a high efficiency in terms of nitrogen oxides reduction while maintaining a low ammonia slip from the tailpipe. Fuzzy logic was used because of the following: the SCR system is a nonlinear, multiple factor dependent system, and the information causing ammonia slip, for most cases, is not clearly defined. An ammonia sensor installed downstream of the SCR catalyst can be used as a straightforward method for potential ammonia slip detection. However, from the cost efficiency perspective, introducing a smart ammonia sensor would greatly increase the cost of the system. The proposed fuzzy controller system collects information from the existing system sensors then incorporates it into the Matlab/Simulink environment. The paper presented the SCR system architecture, diesel emission fluid (DEF) dosing strategy as well as the detailed algorithm of ammonia slip detection with results and comparison to the conventional method.