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
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.
EGR Valve Operation And Description