The exhaust gas recirculation (EGR) system is used to lower oxides of nitrogen (NOx) emission levels caused by high combustion temperature. It does this by decreasing combustion temperature.
The main element of the system is the EGR valve. The EGR valve feeds small amounts of exhaust gas back into the combustion chamber. With the fuel/air mixture thus diluted, combustion temperatures are reduced.
If a problem with the EGR system will not allow the PCM to control pintle position properly, DTC P0404 should set. The PCM also tests for EGR flow. If incorrect flow is detected, DTC P0401 should set. See DTC list for other EGR codes.