Oxygen senosr codes

2007 SUBARU Turbo Outback - Already had rear O2 sensor replaced 5 times and sensor harness. Mechanic cleaned & tightened connections from rear sensor up to ECU which is good. 30 minutes off the dealership lot the “Check Engine” light comes on AGAIN.

The code has been “Low voltage” but the car runs perfect with NO change in performance.

Other than asking for the specific code numbers that are involved, I would say that anyone who replaces an O2 five times is pretty much a wild guesser to put it politely.

In my opinion the first step when it comes to O2 sensor or converter codes is to verify with a vacuum gauge that no manifold vacuum leak exists as a vac. leak can create a false O2 code.
This verification takes only seconds so weed that out first rather than throw a case of parts at the problem.