so the CEL came on and i went to checker auto and got a reading. P1401 came up. i talked with the guy there and he said that i should check/replace the EGR valve position sensor and/or the DPFE sensor. first, what are those and what do they do? second, is the checker auto guy correct? and third, becuase the car did not run any different when the CEL came on than it did before, is this an urgent problem or can it wait a bit? thank you in advance for all the help.