Circuit Description

The DTC P0400 Exhaust Gas Recirculation (EGR) System diagnostic monitors the operation of the EGR valve. The EGR system controls the formation of oxides of nitrogen (NOx) emissions by recirculating the exhaust gases into the combustion chamber. The Powertrain Control Module (PCM) uses the expected changes in the Manifold Absolute Pressure (MAP) sensor readings in order to evaluate the performance of the EGR system. The PCM sends a signal to the EGR valve while monitoring the MAP sensor signal. The PCM suspects that the EGR valve flow is incorrect when the monitored MAP sensor pressure change is too small.