The engine has a lean code and the upstream oxygen or AFR sensor reports lean. It is tempting to replace the sensor, but first ask whether the engine might actually be lean. Check fuel trims at idle and at higher engine speed. If trims are strongly positive at idle but improve with RPM, a vacuum leak becomes more likely. If they remain high under load, fuel delivery or MAF measurement may deserve attention.
Introduce only safe, approved diagnostic changes. A smoke test can reveal intake leaks. Fuel pressure can be compared with specification. MAF data can be checked for plausibility. Exhaust leaks upstream of the oxygen sensor can draw in air and falsely suggest a lean exhaust stream even if combustion mixture is not lean.
If mixture changes are made during professional testing, the sensor should respond promptly. A sensor that remains fixed despite a verified rich/lean change becomes more suspect.
The important lesson is that an oxygen sensor often reports the result of another fault. Diagnose the mixture, not just the messenger.
Figure: Case Study: Fuel Trim Says Lean – Is the Oxygen Sensor Bad?