Technical solutions are described for an emissions control system for a motor vehicle including an internal combustion engine. An example emissions control system for treating exhaust gas in a motor vehicle including an internal combustion engine. For example, the emissions control system includes a selective catalytic reduction (SCR) device, an NOx sensor, and a controller that is configured to detect a NH3 slip of the SCR device. The controller detects the NH3 slip by modulating an engine out NOx from an engine, demodulating the engine out NOx from the engine to original state, and measuring NOx upstream and downstream from the SCR device after the modulation. Further, the controller determines the NH3 slip by comparing gradients in the NOx measurement with one or more predetermined thresholds.