rainy day poco

99 F150 V6 auto. Can anyone tell me what causes these codes
P0401 Exhaust Gas Recirculation Flow Insufficient Detected
P0171 System Too Lean (Bank 1)
P0174 System Too Lean (Bank 2)

thundercougar

P0171 and P0174 together usually indicate a vacuum leak.and that is most likely the cause of P0401 also.but P0401 could be egr valve,egr solenoid,dpfe sensor.gotta trouble shoot that one.but i would look for vacuum leaks first.

kuntrybo

i had the same code a while back, look under throttle body right in front, there will be a rubber elbow, about 3/4 inch in diam. that rubber elbow is replaceable, mine had a hole in it, if that isn't it, run your engine and feel around all the vaccum lines, mine also had a hole in a line coming from pcv valve to the back of throttle body, i just went to local auto parts store and bought some vaccum line and replaced it, it cleared all my codes and havent had any trouble since, thanks i hope this helps.