BBH02S22 Codes P0161 & P0157

Hi guys I am new to the site. I recently got a 2002 Pontiac Trans Am WS6. It is all stock except for the hooker header, slp exhuast & cat. The SLP exhuast and cat where just installed in the last month and the car place it was taken too said the 02 sensors looked ok. RIGHT...Anyways i got the SES light come on shortly after i bought it. car runs fine, idles okay, but gets some terrible gas mileage. Can anyone tell me what these codes mean? I was advised to replace the 02 sensor on the passenger side pre-cat

p0161 - excessive draw or open circuit in the o2 sensor ... make sure your o2 sensor is clean first of all, that is very important with o2's ... otherwise check your wires & connection, make sure everything looks good and not touching any exhaust components - if it does, you may need an o2 sensor

p0157 - make sure you're not having any lean conditions (check fuel PSI, MAF, no vacuum leaks, dirty air filter, ect) and check all wires/connections as well as make sure the o2 isn't dirty. this is the same o2 sensor as the first code so if all of this checks out, replace the o2

** if you're getting bad MPG, definitely check fuel PSI at the rails, filter and pump **

161 is a heater code, 157 is a low voltage code - both relate to the POST CAT sensor on the passenger side. You can look up the codes for your year at gearchatter.com