The urea injection is a key function in Urea-SCR NOx reduction system. As the tailpipe NOx emission standard becomes increasingly stringent, it is critical to diagnose the injection faults in order to guarantee the SCR DeNox functionality and performance. Particularly, a blocked injector may cause under-dosing of urea thus reduced DeNox functionality.Monitoring urea injection rate is one of the efficient methods for injection fault diagnosis. However, direct measurement of the urea mass flow is not feasible due to its high cost. This paper presents methods that are promising for detecting and isolating faults in urea injection by processing certain actuator signal and existing sensory measurements, e.g., the injector Pulse Amplitude Modulated (PAM) command and the pressure of the urea delivery line. No additional dedicated sensor is required.Three methods are discussed to detect urea injection system faults. First, frequency analysis reveals the relation between the shot-to-shot injection frequency of the urea injection PAM command and power spectrum characteristics of the line pressure. A properly working injection system is characterized by a common carrier frequency for both the PAM control signal and the line pressure. Based on this criterion, a fault detection method is developed and could be implemented by using a point FFT algorithm for power spectrum computing. In the second method, a rationality check is proposed to monitor the proper calibration or generation of PAM control signal. The third method uses time domain analysis, two quantitative measurements are generated from the line pressure and the injector PAM signal. The measurements have shown that the magnitude of line pressure variation in time domain provides a good approximate to the rate change of the urea injection flow during transient operations. If a mismatch occurs, a urea under-dosing or over-dosing fault is detected. The proposed diagnostic methods have been verified by vehicle tests equipped with a urea SCR system.