I tried searching for similar threads. 2006 Camry 2.4L. Had code P0138, id'd as the rear (behind cat converter) O2 Sensor. Replaced the unit and CEL went out. Drove 10 miles and the code reset. Any one have ideas?

 

http://engine-codes.com/p0138_toyota.html

There are other things that cause that CEL code to show up

 

You replaced the sensor after the first converter built into the exhaust manifold but before the second converter under the car?

What brand and part number of sensor?

 

No it was the last o2 sensor in the string, behind the cat. Other searches point to possible PCM communications problems. Looks like a trip to the dealer.

 

Does the engine have two cats, one being built into the exhaust manifold and another under the chassis?

If so the engine may have 3 sensors.

P0138 is the 2nd (middle) sensor.