Ford Repair: SES light with P0401 code, vacuum lines, dpfe

Question
My daughters 2001 Ford Mustang had a Check Engine light. I got a scanner for OBDII It gave me a P0401 (Flow Insufficient) code. I replaced the EGR valve. 5 days later it is showing the same code. Where do go next, replace the exhaust gas pipe of the vacuum lines or the sensor? Or all of the above $$$$$,?
Thanks
Dean in Houston

Answer
First, let me know the engine size. It is possible for the pipe to have a defect, or to be collapsed. Another possibility, the more likely, is a faulty DPFE also known as the EGR Valve Pressure Sensor. It is around 30 dollars, and I would start with it.