Changing Your 02 Sensors
By: Bob_98SR5
Written: 8/27/04
Application: 3rd Gen 4Runners

Toyota 02 Sensor Front/Bank 1 (No part numbers given because of make/year/model issues)
Toyota 02 Sensor Rear/Bank 2 (ditto)
Tools & Supplies
Adjustable-head Rachet with 10mm and 12mm sockets
Pipe Wrench
PB Blaster (Liquid Wrench or Tabasco sauce)
Eye protection
Hydraulic jack and jack stands


I am providing this write up for illustrative purposes only. Perform at your own risk. Any mods (including this one) you perform on your vehicle is your responsibility. Commercial use of this write up is prohibited and may result in legal action.

An 02 sensor is a device that measure the oxygen content in your exhaust gas during the combustion process. The data is transmitted to your 4Runner's ECU and the fuel to air ratio is adjusted accordingly. Over time, the 02 sensor's useful life starts to deteriorate and give incorrect data which can result in too much or to little oxygen delivered. The first sign of failure is a check engine light (CEL). The most common of the two sensors to fail is the front 02 sensor, which is located in front of the catalytic converter.

Note: the pictures in this write up was done on a 97 4Runner V6 with 2WD. I did this maintenance work on my friend's vehicle because he failed his smog test. Hopefully this clears that problem up!

Step 1: Prep Works Makes All the Difference

After jacking up your vehicle, spray a generous amount of PB Blaster on the two retaining nuts located on either side of the 02 sensor. Let it soak as long as possible. For you East Coasters, best to let it soak overnight, especially for those folks who live in areas where bad road conditions cause a lot of rust on undercarriage parts.

A good tip: unfasten the 10mm bolt which holds the 02 sensor plug as shown in the picture below. This will allow you to make more room for your adjustable head rachet as well as allowing you to easily unclip the 02 sensor harness:

Front 02 Sensor (O) is shown atop the exhaust pipe to the left of the catalytic converter. Nuts (N) need to be removed.   Remove bolt (B) with a 10mm socket. This makes working on harness (H) easier

Step 2: Removing the 02 Sensor and Plug
Take your adjustable head rachet with a 12mm socket and remove the retaining screws. Next, pull the plug downwards and take your pipe wrench and squeeze the clip from the backside. Pull off the 02 sensor harness
. Pull both the 02 sensor and the 02 sensor gasket out.

Tight space: Use an adjustable head rachet wrench with 12mm sockets to remove nuts (N)   Squeeze the top clip (C) with a pipe wrench which will easily allow you to pull out harness (H)

This is what a spent 02 Sensor and the gasket looks like:

Step 3: Putting Everything Together
Plug the harnesses into each other. Next, place the 02 sensor gasket on. Make sure you place the gasket the correct way. See the picture below:

This gasket faces towards the TOP of your 4Runner and the underside of the 02 sensor   This gasket faces towards the BOTTOM of your 4Runner onto the exhaust pipe. Notice that there is less white material on this side

Next, mount the 02 sensor back onto the stock bolts and then loosely screw on the retaining screws. Fasten these screws down, alternating until both sides are fastened down. Follow up by fastening the plug with a 10mm socket.

That's it. For the rear 02 sensor, it is much easier due to the fact that there is alot of hand space to access the 02 sensor itself as well as the 02 sensor harness.

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