BrApple - I am correct because it happened to me and these cars with the old style sensor can exhibit this error, P0401 can be the DPFE sensor failure along with the 1400 error codes, or it can be the EGR solenoid, EGR valve failure, or blockage.
"A code P0401 most likely means one or more of the following has happened:
- The DPFE (feedback EGR) sensor is faulty and needs to be replaced
- There is a blockage in the EGR (tube), most likely carbon buildup
- The EGR valve is faulty
The DPFE senor will comunicate to the PCM and the PCM along with other function will activate the EGR solenoid that activates vacuum to the EGR valve.
Try and put a vacuum on the EGR valve itself and see if the symptonms change.
This is where true understanding of the EGR system comes into play. The codes are not always 100% based on the description.
OBDII continiously monitors the DPFE signal, at idle. Since no EGR flow at idle the DPFE signal should show no flow voltage signal. The PCM compares the Key On Engine Off stored voltage to the DPFE signal at idle If the signal is not consistant with this value it may indicate a stuck open EGR valve.
So depending on the state a P04xx code can be thrown or a P14xx code can be thrown.
The P14xx code will be thrown if a negative voltage is detected, or inconsisant values are seen from the DPFE sensor.
So never say never!