A detection circuit for detecting faulty operation of an error correction code (ECC) decoder that is configured for diagnosing whether an error has occurred in input data to the ECC decoder, wherein the ECC decoder is further configured for outputting an error detection signal indicative of whether the error has been detected and potentially corrected by the ECC decoder and output data based on the input data, and wherein the detection circuit includes a first stage configured to generate a first check signal indicative of whether there is a mismatch between the input data and the output data of the ECC decoder, and a second stage configured to generate a second check signal indicative of whether faulty operation of the ECC decoder has been detected based on the first check signal and the error detection signal of the ECC decoder.