Service engine light is back on and still saying o2 censor but just replaced both of them ford zx2 2002?

A code P)171 doesn't say to replace the O2 sensors. What it means is that Bank 1 is lean. Could of been a bad O2, but most likely its from either low fuel pressure, a bad MAF or the most common, a vacuum leak.

Yes. You relied on the OBD scanner for Diagnosis and fixed the reported SYMPTOM instead of the problem.

What was the Specific Code(s) that the scanner told you...

The OBDII codes tell you what DATA on what circuit is out of range...NOT what device is bad.

As you found out the OBD codes are correct 40-60% of the time but who wants to pay for 40% error rate.

Use the codes to Diagnose what conditions could cause the problem being reported by the codes, or better yet take your car to a Licensed Emissions Repair center mechanic and pay them the $100.00 diagnosis fee to have them spend an hour + testing your car to find the problem. Certainly less expensive then replacing two O2 sensors that didn't need to be replaced.

And remember with a Free "diagnosis" like you already "paid" for, you do "Get What You Pay For"

8 Years licensed Emissions test & Repair Tech and Station Owner

We really need to know what the code is. There are a lot of codes related to O2 sensor circuits and heater control circuits that are rarely in the sensor itself. The first source has some of the more common causes of many of the more than 30 O2 sensor codes. However, at 9 years old the O2 sensors were about due for replacement anyway.