P0400 is for EGR (Exhaust Gas Recirculation) Function

This system cuts and controls vacuum applied to the EGR valve to suite engine operating conditions. This cut-and-control operation is accomplished through the ECM and the EGRC-solenoid valve. When the ECM detects any of the follwoing conditions, current flows through the solenoid valve, this causes the post vacuum to be discharged into the atmosphere. The EGR valve remains closed.