Circuit Description

WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The engine control module (ECM) uses the mass air flow (MAF) sensor signal to calculate the amount of exhaust gas recirculation (EGR) flow. The ECM will control the EGR valve vacuum control solenoid and the EGR vacuum vent solenoid to allow vacuum to operate the EGR valve. The EGR control system uses vacuum from a belt driven mechanical pump called a vacuum pump. Diesel engines do not create enough engine vacuum on their own to allow the EGR gases into the combustion process. When the ECM commands the EGR valve to open, the EGR throttle valve will be commanded closed. The EGR throttle valve creates a restriction of incoming fresh air to the engine in order to create engine vacuum. If the ECM detects a calibrated difference between the desired MAF rate and the actual MAF rate during EGR system operation, DTC P0401 sets.