P0404 - Exhaust Gas Recirculation Circuit Range/Performance
If you have a P0404, then the EGR valve is likely an electrically controlled EGR valve instead of a vacuum controlled EGR valve. Also, the valve will usually have a feedback system built into it that informs the PCM (Powertrain Control Module) what position the valve is in; open, closed, or somewhere in between. The PCM needs to know this to determine whether or not the valve is operating as needed. If the PCM determines that the valve should be operating, but the feedback circuit shows that the valve is not open, this code will set. Or if the PCM determines the valve should be closed but the feedback signal indicates that the valve is open, this code will set.
Usually this code points to either carbon buildup or a bad EGR valve. However that doesn't rule out the following:
Open or short in the 5 Volt reference circuit
Open or short in the ground circuit
Open or short in the PCM controlled voltage circuit
Bad PCM (less likely)