O2 Sensor replacement on 02-07 Subaru WRX/STi:
This is a guide to replace your front 02 sensor in your 02-07 Subaru WRX/STi. This guide will go step by step through the process.
Denso 234-9120 Oxygen Sensor (Air and Fuel Ratio Sensor)
1.) The first thing you need to do is place the jack under your car. I chose to use the pinch weld under the passenger’s door area. Next you will want to remove the wheel from the car so you will have better access to the O2 sensor. I did this by using a 19mm socket with an extension.
2.) Upon removing the wheel, you should notice a small plastic flap with a few plastic retaining screws holding it in place. I have circled them in red in the picture below.
3.) You are going to want to remove these using flat head and phillips head screw drivers. Take care in making sure you do your best to not strip them out. If you do, you will end up having to either pry them out or pull them out with a set of pliers. If you need to replace any of these, you can usually find them at any autoparts store.
4.) Once you have all of the plastic screws pulled out, remove the plastic cover.
5.) If you pull the remaining plastic cover to the right, you will be able to see the O2 sensor. You can opt to remove the larger of the plastic fender well shields, but I did not see much point in doing so as it would not really yield much more room for access.
6.) With the O2 sensor exposed, you will want to spray it with some lubricant and my preferred lubricant is PB Blaster. With that now soaking in a bit, you are now going to want to go and pop your hood to gain access to the plug end of the sensor.
7.) Looking at your engine bay, you will want to move to the left side (passenger side). Underneath this cover are two connections, the top one is for your front O2 sensor. Go ahead and disconnect that plug.