The paper addresses spot defects in CMOS RF blocks. In a softer resistive form they degrade the circuit performance such as noise figure (NF) and gain. The NF considered here is the test response. Based on a noisy two-port model an impact of the generic faults is analyzed. A practical example of a CMOS mixer supports this analysis. Discussed is also effect of masking by the circuit tolerances. The detectability threshold for the generic faults is captured assuming Gaussian distribution for the two-port parameters. The analysis reveals both strength and weakness of the presented test approach.