I have a trouble code P0133 that says 02 Sensor Circuit Slow Response (Bank 1 Sensor 1) I know I need to replace an 02 sensor but I am not sure which one. The parts store is asking if it is before or after the cat converter. Can someone help.

 

Bank 1 is the engine bank with #1 cylinder.....the passenger's side.
Bank 2 is the driver's side.
Sensor 1 is the front one, sensor 2 is after the converter.

 

Thank you for your help Racerguy. I got started down a wrong path here and have two different post. I wont do that again but I am wondering if I have a vacuum leak rather than a bad sensor. I hear a pop once and a while from inside the cab that I am assuming is a backfire and the truck shifts hard periodicly. It also has a mild spark knock under pressure. Any advise