have a 2001 crown Victoria lx,,,has a 401 code could it be the egr vavle or dpfe.....also u hear a almost high pitch sound when heat or ac is on from inside the car .....air leak?

Yes, check for a vacuum leak FIRST that can trip egr codes

changed egr valve and also the dpfe .....sensor.....it was actually melted on one side ......what would cause that it loooked like a gernade went offf on one side....car is running smooth now no hesitation or stumbiling ....