'02 Mitsubishi Galant P0135/p0136

I just got this car and the check engine light came on...The code was either P0135 O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1) or P0136 O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)

I know what where and how to change the O2 sensor, I just don't know which one to replace, the one before the cat or after. I'm guessing bank 1 is in front of the cat and bank 2 is after. Just wanted to doubble check with some people who know.

First, what size engine do you have? If it is a four cyclinder, then there is only one bank. If it is a V6 then bank 1 is the side of the engine that cyclinder #1 is on. When the description of the code (P0135) is bank 1, sensor 1 it means the O2 sensor (pre cat) on bank 1. Bank 1, sensor 2 is after the cat. This sensor is called the cat monitor. Some vehicles with V style engines will have the exhaust from both banks routed into the same cat, so these vehicles will only have on cat monitor.

If the mileage is high enough I'd recommend changing both, they can wear out without ever throwing a code and you're gas mileage will probably improve