service eng light on 1998 Nissan Sentra

I have this light on in my dash, end i do diagnostic test, end
the answer was: low flow detected, end 4 probable cause:
EGR-passages blocked
EGR-pintle stuck closed
EGR-volume control valve open or shorted
EGR-temperature sensor error.
So; my cuestion is, how i know wich is the real cause of my service eng light.

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1 answer
Sounds like the EGR passages are blocked and causing the check engine light to come on New EGR valve is $150 to $200 at the parts stores but its $107 at http://www.parts.com/