[DRW1000]

The EGR valve opens when the engine is warm and at relatively high RPMs (I think about 2500 and up). This valve routes some exhaust gases into the intake stream and lowers the combustion temperatures and thus reduces NOx production.

If you are getting P0401 then the PCM "knows" it has commanded the valve open but the feedback from the DPFE indicates that there is low flow.

This could be caused by the EGR vacuum regulator (solenoid?), the EGR valve itself, the DPFE or even clogged ports.

One easy way to check the valve operation and the ports is to place a hose on the EGR valve vacuum line (disconnect the one already there and plug with a golf tee). Whith the engine idling gently apply vacuum to this line via a vacuum pump or mouth power. If your EGR is working it will stall the engine. If some of your EGR ports are clogged then usually you will have a misfire (different codes) on the least clogged port's cylinders.

You can test the DPFE by using the same technique while measuring the voltage from the DPFE (backprobing). You will notice the voltage is proportional to the amount of vacuum applied.

If these items seem to be okay then the problem may be with a clogged vacuum line or the vacuum controller itself.

None of these should cause strange behaviours unless a misfire code appears too. An inoperative EGR system should give the code you mentioned but I don't believe the van will run differently.