The EGR (Exhaust Gas Recirculation) valve is used to reduce NOx (Nitrogen Oxides) emissions. To do this, it feeds small portions of exhaust gas into the combustion chamber. It is worth mentioning that NOx (Nitrogen Oxides) is caused by high temperature. High temperature is reduced when the air/fuel mixture is diluted with the exhaust gases.
The design of the EGR (Exhaust Gas Recirculation) valve allows exhaust gases to be accurately supplied to the engine without using the vacuum in the intake manifold. Exhaust flow into the intake manifold is distributed by the EGR valve through an orifice that controls the PCM (Powertrain Control Module).
If you have the DTC code P0404 OBD2 set, this means that the EGR valve is electrically controlled. This valve has a feedback process, which notifies the PCM (Powertrain Control Module) of the position of the EGR valve, i.e. whether it is open, closed, or perhaps somewhere in between. This data is required by the PCM, as this provides proof of the EGR valve's function.
In case the PCM decides that the valve must be running, but the feedback loop indicates that the valve is not open, (applies in the opposite direction as well) the fault code P0404 is set.