How To Diagnose And Change Your Cars O2 Sensor

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    If your car has a loss of power and or gas mileage and it has over 60k miles there is a pretty good chance that you will need to perform a full tuneup which may include an O2 Sensor replacement.

    Although most O2 Sensors won’t actually go bad after only 50k to 100k miles they will get covered with carbon deposits that make them inoperable. I have seen some people try to clean them with a torch and this might work for a short period but like cleaning spark plugs its probably not worth the effort.

    On older vehicles you will find a single O2 sensor that is mounted on the header manifold that connects directly to your engine. On newer vehicles you will have two O2 Sensors. One that is located on the Manifold and one that is located on your catalytic converter.

    The difference between new and old is that the first O2 sensor on the manifold tells your car how to run based on burnt fuel levels. The sensor that is on the catalytic converter tells your car how much emissions is getting past the Catalytic Converter and will adjust your fuel air mixture to improve emissions vs performance.

    Diagnosing A Bad O2 Sensor

    When your O2 sensor is going bad you will find that you start losing gas mileage. The amount of Miles Per Gallon can be pretty dramatic on a small vehicle that normally gets really high mileage in the range of 28 to 40mpg you might lose 5 to even 15 miles per gallon as the O2 sensor begins to go bad or builds up carbon deposits.

    Once the O2 sensor just can not work anymore you may find that your car sends you an Amber Check Engine light on your dashboard. This might be intermittent or it might be constant. The best way to diagnose a bad O2 sensor is to check your computer codes when the O2 Sensor causes a Check Engine Alert.

    NOTE: When your vehicle’s computer tells you that your O2 sensor is bad sometimes it can be something else. Your O2 sensor is just that a Sensor of whats happening. Think of it like your Speedometer. Your Speedometer doesn’t control the speed of your vehicle it only reports the speed you are driving. For this reason you could replace a good sensor and still have the original problem.

    The vehicle I am working on is a 1990 Honda CRX and since it is an older vehicle it only has a single Up Stream O2 Sensor the procedure will only be half the work of newer vehicles that come with both a Up Stream and Down Stream Sensor.

    When my O2 Sensor goes bad I will receive a Check Engine Light and then my computer diagnostic will read a Code 1. The Code 1 means there is probably something wrong with the O2 Sensor and that you should check it.

    If you have a newer vehicle with two O2 sensors the Code 1 from the ECU will normally have a Code 1a and a Code 1b telling you which sensor is causing the Check Engine condition. 1a would mean the Up Stream O2 Sensor and 1b would mean check the Down Stream Sensor at the Catalytic Converter.

    Each Manufacturer is different and you can find a Code list for your specific vehicle in your vehicle’s Service Manual.

    My vehicle’s ECU has a Diagnosis LED that blinks the Code but if you have a newer vehicle you will need a Code Tool that plugs in to a port under your dashboard.

    If you do not want to buy a Code Retrieval Tool then most Auto Parts Stores will do the test for free.

    Once you have the Code you should check your Service Manual for the specific Diagnosis Procedure. Like I said the O2 Sensor is a Sensor not a Regulator so the Diagnosis Procedure will tell you other things to check that could be causing the problem.

    In my case my service Manual says that the Fuel Regulator could also be the problem and to check it you would need a fuel pressure tester that connects to a service bolt you remove from your fuel filter.

    Your Manual will have other items you should check.

    Replacing Your Bad O2 Sensor

    Since I have never replaced my O2 sensor and my vehicle has over 150,000 miles I figured it was time to do so. My fuel mileage had been lower than normal and I was having problems with emissions.

    The cost of a new O2 sensor was not that bad only about $20 with one that had an OEM Exact Fit electrical connector.

    I strongly suggest that you pay the extra few dollars for a sensor that has the Exact Fit Connector instead of a Bare Wire that you need to splice to your old Wire. Splicing normally goes ok but in this case you are splicing a wire that is in harsh and hot conditions. Additionally depending on how much room you have to work splicing the wire can be very difficult.

    Newer vehicles with more than one wire on the O2 Sensor use the additional wire for a O2 Heater. This heats the O2 Sensor when you first start the car until the exhaust temperature can take over. If you mis-splice these wires you can cause damage to your vehicle and possibly blow out a computer. It is not worth doing this just because you wanted to save $5 and splice your own wires.

    Make sure you disconnect the battery cables before you begin.

    Removing the O2 sensor can be difficult. If you are lucky the sensor is easily accessible but even if it is the heat of the exhaust may have froze the sensor to the manifold.

    You will need a special O2 Sensor Socket. Most parts stores will provide you with this tool as a rental but you can also purchase them for about $10.

    To remove the sensor you will at the very least want to use a Breaker Bar which is like a socket wrench but it does not ratchet. If you use a ratchet wrench you have a high probability of stripping the gears and breaking the wrench.

    Because my sensor was in a very difficult position I had to remove my passenger side radiator fan and use an Air Powered Impact Wrench to remove the Sensor. I suggest that if you have access to an impact wrench that you use it because it will make removing a seized sensor much easier and you will reduce the chance of damaging the threads.

    Before I used the impact wrench I taped a small piece of 1/8th inch luan plywood to protect my radiator. Radiators are sensitive to damage so a slip of the wrench whether an Impact Wrench or a standard wrench would definitely cause damage. Since I just replaced the Radiator I was highly sensitive to this but it only takes a minute to tape a piece of plywood in there.

    Remove the electrical connector before you begin so that you do not damage it.

    Place your impact wrench on its lowest setting and a couple pulls of the trigger should remove the O2 sensor.

    Before you install the new sensor you will need to apply antiseize to the threads. It is probably a good idea to apply it to the threads on the manifold and the sensor. Do not use too much. Only a very tiny amount is needed to cover the threads. Normally you will get a small tube with the O2 sensor. You should keep the rest for other jobs. The tube I received seemed to have copper metal in a paste other types will have aluminum.

    When installing the new sensor you want to make sure not to torque it too tight so use a standard ratchet wrench to install it. You can also use a torque wrench.

    After it is installed you can attach the connector. Normally these connectors will only install in one direction. If you can inspect the receiving side to make sure its clean.

    Now that the O2 sensor is installed you can assemble any other parts that you needed to remove to access it. In my case i have to reinstall the passenger side fan on the radiator and the top radiator hose.

    Reconnect the battery cable and you are done.

    You should now run the car at idle for about 10 minutes before you drive it.


    Final Note

    Although your O2 Sensor should begin working immediately it is not unusual for it to take 50 to 100 miles of driving before it works correctly. If you are replacing the sensor to pass inspection make sure that you take that time to drive the car before you take it in for your test.

    The total cost for this repair was about $20 however I completed other work at the time this procedure was completed.

    The up stream sensor is something that most people can replace because most new vehicles give easy access to it unlike mine.

    If you don’t feel you are up to the job because you have to remove too many parts to get to the sensor or you don’t have the tools a repair shop should be able to perform this work for you in about a half an hour of labor.

    Remember the O2 sensor is just a sensor that helps your computer control other devices on your engine. If there are other parts that need repair even something as small as a leaking vacuum hose you can get a false positive on this test and end up throwing lots of parts at your car only to end up needing a $1 piece of vacuum hose.

    Read your service manual and ask for help if you need it.




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