I have a 96 Contour with the 4 cyl. It has about 70K on it. I have replaced both O2 sensors about 2 years ago when the check engine light came on (guessing). It fixed the problem. The light came back on about 6 mos ago. I took it in and it had a dirty mass airflow sensor. The light came back on. I bought a fault code reader yesterday and used it. I get fault code 171 which is "system too lean bank 1". I have no clue what to fix. I see something that says "rear" O2 sensor, but that shouldn't have failed so quickly. Right? Does this mean that I just need lots of fuel cleaner additive? If so what works best. Every product you see says it's the best.
Help!
Steve.gould@apawood.org 
