If the 02s are the orginal or your not sure how old they are for the cheap cost I would replace both fronts but again I'd check the fuse for the 02 heaters and assure there is no exhaust leaks or wiring problems.

With scanner I would do the I/M and 02 tests and see if they pass.
If the scanner supports the mode 6 tests I also would run it and look at the results