po 401 egr
 

I have a 2002 f-150 4.2 L engine standard I scanned the code and it comes up PO 401 egr flow insufficient, I have had it in the shop 3 times for this to pass inspection which it wont with the service engine light on.I have changed the egr valve twice and the light goes out for about 50 miles then back on . Any help would be appreciated Thank You all the hoses appear ok I recently replaced the coil.