EGR Solenoid Vacuum Valve
WARNING: This page is about a different car, the 1996 Suzuki Esteem. However, it is still accessible from the selected car via links, so may be relevant.
See DTC P0400 in appropriate BASIC TESTING article. Since many computer-controlled and monitored components set a Diagnostic Trouble Code (DTC) if they malfunction, also perform procedures in appropriate TESTS W/CODES article.