Tag Archives: o2 sensor

O2 Sensor replacement on 02-07 Subaru WRX/STi:

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)

O2 sensor location in the exhaust manifold.
O2 sensor location in the exhaust manifold.

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.

Engine Management For Subaru WRX/STi


Why do I need engine management?


Consider your stock engine management for just a moment. Your stock engine control unit (ECU) is a very complex piece of circuitry that calculates hundreds of variables every second. All of these variables rely on inputs within a + or – range. When you modify your vehicle, these values change. As long as the changes are within the values the ECU expects to receive, your engine runs fine. Once the values are exceeded, the ECU is programmed to compensate to return the values to normal levels.

A typical way of engine management is to use a Cobb Accessport.
A typical way of engine management is to use a Cobb Accessport.

This is a layman’s explanation of how your stock ECU can actually work against you when modifying your vehicle. This also explains why modifications can feel great once they are bolted on but the butt dyno results seem to fade over time. This is due to ECU compensation.

What is the first step in finding what engine management I need? Finding a tuner. The Tuner FAQ will help with the general rules of finding a good tuner. Remember, it’s always better to have a custom tune vs. a plug and play or “staged” map. Always defer to the tuner’s advice as to what to choose as ultimately he will be the one to provide custom support. Discuss your goals and budget and your tuner should set you on the right path. If you are a “plug and play” kind of person, review the options below and decide for yourself along with input from locals in your regional forum and the car parts review forum.

What will engine management do for me? Generally speaking, engine management optimizes several engine functions to create more horsepower and efficiency. The stock ECU is designed to ensure your car runs fine and monitors the engine’s output parameters. Utilizing an aftermarket engine management solution takes this to the next level.