This correspondence presents a totally self-checking error indicator for solving the practical problem of monitoring the TSC checkers. We consider TSC checkers designed for checking coded information (data or address) or periodic signals with tolerable transient failures smaller than a constant time limit T, which depends on the propagation delay of the used logic elements. This circuit provides normal indication outputs (01) or (10) and error indication outputs (11) or (00). Permanent failures or transient failures with time duration greater than T result in a permanent error indication, which remains until reset. Provided that an assumed fault occurs within the checker, the permanent error indication output is produced at most after four input changes from (01) to (10) or from (10) to (01).