I recently got the p0138 code. what should i check for? should i replace the rear o2 sensor?

 

No. Chances are it's being caused by your test pipe. Are you running an o2 simulator or a spark plug non fouler?

 

Your front o2 converts it's signal into voltage. Less than 1.0. Usually around .9xx. The rear HAS to read lower (think catalytic converter) or it will throw the check engine light. The spark plug non foulers can be purchased at any local auto parts place. They are maybe $5 for two. You have to drill out the inside with a 1/2" bit for the o2 sensor to screw into properly. Then screw that non fouler into your exhaust and you should never have another check engine light from it.

 

i'm running a cel fix for the test pipe o2 sensor