In order to achieve that the EGR valve circuit can meet the emission requirements of the on board diagnosis (OBD), based on the syncretic theory of the principal components analysis (PCA) and the echo state network arithmetic (ESNA), an on-line diagnostic strategy on the circuit fault of the diesel engine EGR valve is proposed. The fault in the EGR valve circuit are simulated by PSpice, the sampling data are regarded as the inputs of the PCA, the fault codes on the circuit components are regarded as the outputs of the ESN, the diagnostic strategy in the EGR valve circuit for diesel engine is educated and studied by PCA-ESN. The on-line diagnostic tests are made by the PCA -ESN syncretic theory. The test results show that the diagnostic accuracy can also reach 94.3 %, the errorrateis 0.58%, and the rejective rate is 0.15%, thus it testifies that the diagnostic strategy can carry out the accurate diagnosis and the orientation of the malfunction on the EGR valve for diesel engine, and completely satisfies the EGR valve start under various conditions.