2003 Toyota Highlander Q&A
2003 Toyota Highlander Question: OBD II shows front upstream O2 Sensor is bad.
was it 'Toyota' or aftermarket 'parts' that asked you that question?I'd bet it was advance or autozone. It's an a/f sensor pre-cat , O2s , after (or in middle) of cat (downstream). one is for engine management the other is for emissions monitoring.(short explanation). Safer and less costly to keep your Toyota , all Toyota!! - ziptie12