I bought a used 2011 es350 a year ago. It has 51,000 miles and threw this code a few weeks ago. I replaced bank 1 sensor 2 O2 sensor. I unhooked battery and code went away for 40 miles then reappeard. I disconnected battery again and 40 miles later check engine light back on with same po138 code. I have a new air filter and all the wires to sensors looks good. The gas is from November and I was wondering if stale gas can be my problem as I don't drive the car much in the winter. Any ideas on what the problem is? The car is running like new. No miss, 26.5 mpg which is what I always got since I bought it with 36,000 miles.

 

Just a heads up for others who may get the PO138 code with nothing else. I found this article( http://m.autoserviceprofessional.com/article/95971/lexus-a-f-and-o2 ) and it was exactly the voltages I was getting. I replaced the Bank 1 sensor 1 after replacing Bank 1 sensor 2 didn't help. After replacing sensor, I cleared the code but after driving for awhile the same code was found in "pending codes". After driving another 5-10 miles the code cleared itself and I've driven probably 30 miles and still no codes so I'm cautiously optimistic.