A couple of weeks ago I got the dreaded 0171 and 0174 error codes. After much searching it appeared that it was either the PCV Hose at the back of the intake or the EGR flow sensor. I bought everything I needed to replace the EGR sensor and PCV hose. Wanting to be systematic in my efforts (and be able to return items I didn't need) I swapped out the $1.50 PCV Valve first. Bingo two weeks later and stil no codes. If you get the 0171 and or 0174 code try your PCV Valve first.

Anyway, thats my $0.02