Originally Posted by leechris
p0138 is rear o2 high voltage right?
I believe so. Is is definitely an issue with the post-cat sensor. The challenge is finding out why the code is being thrown. Just replacing the sensor does not always fix the error. Iit can be anything from the sensor wire grounding itself somewhere upstream to a bad converter. In my case, my cat was original with almost 200,000 miles and rusty. It was a safe assumption it probably wasn't doing its job anymore. It wasn't worth doing an emissions test to find out, and use the money saved towards the purchase.
I think I paid around $130 online for an OEM replacement. Since I was replacing my whole exhaust system anyway (my o2 sensors fused themselves into the system), I figure the price was worth the peace of mind down the road and might help resell value for people concerned with passing emissions.