Q: Computer reads an issue with the oxygen sensor on a Subaru Forester 1999 2.0 Turbo. What is the problem?

Your 1999 Subaru Forester has pre and post catalytic converter oxygen sensors. The pre-converter sensor optimizes fuel trim while the post converter sensor monitors the efficiency of the catalytic converter. A failing pre-converter sensor will degrade vehicle MPG performance. Although a code may set that "identifies" one of these sensors that does not always mean the sensor is "bad". For example, the post converter sensor may show up in a Diagnostic Trouble Code but the real issue is an actually failed catalytic converter. Similarly, a variety of causes, such as sulfur contaminants from leaking valve stem oil seals, and even a failed fuel injector, can cause the pre-converter sensor to fail. Although you could replace the sensor, it will fail as well unless the underlying malfunction is addressed. For a comprehensive diagnosis of the trouble code, and repair, of this issue feel free to contact YourMechanic for assistance.