I recently got CEL while driving back home the other day. The two codes I pull was :

P0138 oxygen sensor circuit

P0136 oxygen sensor circuit bank 1

I reset the ECU and it the CEL hasn't come back on. I've driven 500 miles already , but I'm getting horrible Gas mileage. Could my o2 sensor gone bad, that I'm getting horrible gas mileage ??
Forgot to add that I have a test pipe,exhaust & headers. Is there any way to check if my O2 sensor gone bad? Thank you

 

I think your hunch is correct. O2 sensors are going bad. Not sure where you can check them but you can try Autozone and other parts store. Maybe cleaning them will fix the issue.

O2 sensors are quite expensive so you want to make sure.

Good luck.

 

Yep, looks like a bad Primary O2 sensor to me

I've bought them from oxygensensors.com before for a decent price.

 

Ok thanks .... I guess I will be ordering one now ....

 

Thanks I knew I remembered you posting a good place to get them a bit cheaper. Search function win!