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Vacumn leak, Bad sensor. The sensor is reading rich and the computer can not adjust the air/fuel enough to change it so it throws a code.
Do you have any runability problems? O2 sensor is not real $ so if its original and you have a few miles on it I would start there.
I pulled a code 172 "HEGO SENSOR FAULT - LEAN" on my '94 F150 302.
After checking the wire harness, I found a corroded lead right at the O2 sensor. I picked up a new sensor for about $40 and haven't had a problem since. Clear the codes and if it reappears you might want to look into a new sensor.
actually, right before this happened i was changing out my starter and i know i hit the harness for the o2 sensor several times with my ratchet and i was moving it all around to get the starter up and down. i was thinking that it may just be a connection. i'll check that out. thanks