So finally I caught my SES non stop; drove to my buddy's shop - grabbed his OBD II shop reader and voila - P0138 and P0139. Bank 1 sensor 2 (O2 sensor) has flactuating voltages at random throttle positions.

Now - two questions:
1. Where specifically is that sensor? I think it is probably the one behind the cat.
2. What's the cheapest best price for a new O2 sensor?

 

an o2 voltage should flucate a little bit. not too much tho. 1.5v is the standard. should go up and down a little bit with throttle.

p0138 p0139 high voltage and slow responce, normally judgeing by that i would replace it. rear is correct. sensor 1 is the pre-cats and sensor 2 are the post cats.

 

try http://www.oxygensesors.com