Diagnostic code P0404 indicates an exhaust gas recirculation (EGR) control circuit range or performance problem. This code is normally caused by carbon build-up around the EGR pintle.
Other possible causes:
Faulty EGR valve, incorrectly installed EGR valve, electrical faults, restricted exhaust, defective PCM.
Use a scanner to compare actual EGR position to commanded position. If readings do not agree with each other inspect the EGR pintle for carbon build-up. If excessive carbon is found, clean or replace the EGR valve. The root of the problem may be carbon accumulating in the EGR passages. Inspect the EGR passages and clean if necessary to prevent recurring problems.
Do not allow deposits to fall into combustion chamber when cleaning carbon accumulation from EGR passages.