One of the easiest things you can do is clean the MAF sensor. My daughter's car posted a 172 and 176 code on a 95 Mystique v6. She is 3 states away. I checked with my local mechanic who had previously worked on the car.
He states that with all Ford products the MAF gets dirty very easy. This changes the resistance through the little wires. The O2 sensors are programmed to accept what the MAF says as fact and this will cause them to post the lean codes. Takes about 5 minutes and the only problem is the torx screws are the anti-theft type.
Posting a link below that shows how to clean but uses another vehicle as an example.
http://www.berrysweb.com/clean_your_maf.htmhttp://www.fordf150.net/howto/clean-maf-mass-airflow-sensor.php 
