2010 Corolla, P0138 code. Downstream O2 sensor.
Downstream O2 sensor. Replaced sensor with new Denso sensor. Cleared
code. Check engine light returns next day same code. Diagnostic tool reads
.9V (rich mixture). Attempted fixes nothing worked, bottle of Cat cleaner, new
spark plugs, new gas cap, cleaned throttle body and Mass Flow sensor,
bottle of fuel injector cleaner. Any suggestions much appreciated.
When a code points to something like an O2 sensor, the problem might not be the sensor itself, but it could be a damaged wire or a loose or broken connection leading to the sensor. That's where a mechanic's diagnostics come in. The returned CEL should have led you to recheck the code; I'd assume it's still the O2 sensor.
Thanks for response. Yes still same code. I sprayed electronic cleaner on connector contacts, will double check sensor voltage with voltmeter, in couple days when weather permits.