O2 SENSORS...

Difficulty? Low

Total time: Minimal.

Applications:  86-88 NA/Turbo, 89-91 NA/Turbo

What does the O2 sensor do?   The O2 sensor is located in the exhaust manifold, and its primary duty is to measure the exhaust gases coming from the engine for the presence of oxygen.  If the sensor detects O2 in the exhaust gases, then it tells the ECU that the car is running lean.  If there is no O2 found in the exhaust gases, then it tells the ECU that the car is probably running rich.

However, The O2 sensor is not an exact instrument and can not be used as a comprehensive tool to measure for the presence of O2 in the exhaust.  It is an approximation of either rich or lean, that's all!  For a comprehensive reading of O2, you would have to take the car to a specialty shop that has the proper oxygen sniffing equipment. $$$$$

Operation of the sensor is pretty simple.  During stop and go driving, the output from the sensor is ignored.  The only time the ECU cares about the O2 sensor is when your cruising at constant speeds.  For example, when your cruising on the interstate.  The ECU then enters what is referred to as "closed loop" operation.  The ECU then monitors various temp sensors, the O2 sensor, the air flow meter, and the TPS in order to change the air / fuel mixture.  It does this in order to obtain better gas mileage and lower emissions.

Mazda recommends changing your O2 sensor every 80,000 miles.  I made it to 120,000 before mine went bad though.  When you do have to change it, I would strongly suggest not buying the Mazda OEM part.  The OEM part costs ~$125 but if you look at it closely you'll see that the OEM sensor is a standard Bosch O2 sensor with a Mazda plug on the end.  You can go to any auto parts store and buy the Bosch plug for about ~$30.  Then all you have to do is remove the plug from your old sensor and solder it onto the new sensor.

DIAGNOSIS:

The first and foremost sign of a bad O2 sensor is BAD GAS MILEAGE!  Depending on your modifications, you will probably notice that your car looses about 4-5 MPG!  In my non-turbo I was used to getting about 19-20 MPG when all of a sudden I was getting 15 MPG.  

To test your sensor for any signal, first you need to unplug the sensor.  If you follow the wire from the O2 sensor up, you'll see that there is a green connector right below the intake manifold.  Once unplugged, take your positive lead from your volt meter and attach it to the sensor, then ground the negative lead to the chassis.   Next, start the car and let it warm completely.  At idle, your meter should read less than .1 volts, usually nothing at all.  However, when you rev the car up to about 2000 RPM, you should see the voltage rise as you add more fuel.  When you let off the throttle, the voltage will drop down again.

If you can get the voltage to rise and fall when your on and off the throttle, then your O2 sensor is working properly.  If you get no readings as you increase/decrease the throttle, then its time to head to the parts store.

REMOVAL / INSTALLATION:

O2 sensors can be a real pain in the butt to get out!  One thing I would suggest it getting a O2 sensor socket when you go to the parts store to buy your sensor.  This O2 sensor socket is a deep socket has a slit cut in the side of it, so you don't break the wire coming out of the sensor.  Also, a "crows foot" at the end of a long extension worked for me when I had to get mine out.

Remember to use LOTS of penetrating oil!  If you strip the old sensor in the manifold, your going to go from bad to worse...  Nuff said!  Remember though, once you remove the old one, Clean the manifold completely of all penetrant!  O2 sensors don't take very well to oil and grease on them.  They will often not work if they become contaminated.

On the installation side, remember to use the anti seize lubricant that comes with the new sensor.  If you don't you'll never get it out again after the next 80,000 miles! (You are planning on keeping your car for another 80,000 miles, right?)