The p0402 code is excessive EGR flow. This code is used to signal a stuck open valve (particle lodged on the valve seat or clogged ambient port).
There are 3 parts to the EGR system. You have the Solenoid, the Valve, and the DPFE sensor. The PCM gives a Pulse Width Modulated signal to the Solenoid depending on how much EGR is desired. The solenoid then converts the electronic PWM signal to a vacuum and opens/closes the valve accordingly. At idle and WOT (Wide Open Throttle) no EGR will be requested.
If I were trying to figure this out, I would disconnect the hose off the valve; reset the codes (pull battery connection or appropriate fuses for 15 minutes,); then drive the vehicle with for a while and see if the code sets. The system will not be delivering any EGR flow if the valve is closed.
The system may then throw a p0401 code which is insufficient flow. If it gives the p0402 code I would focus on the valve or ambient port, if it gives a p0401 I would guess that the DPFE sensor is giving a bogus EPTZER reading and needs to be replaced.
