This paper is to study the validity of the G-diagnosis algorithm that has been regarded as a classical algorithm for faulty diagnosis in analog circuits. The results of this study indicate that: （1）wrong mathematical models are used to estimate G-parameters ; （2）the estimated Gparameters must be biased against actual G-parameters;（3）the G-algorithm will give mistaken diagnosis conclusions if harsh conditions can not be satisfied. The paper proposes formulas

which are easily calcuated with nominal data of circuits

to forecast if the G-diagnosis algorithm can locate faults correctly or not.