Oxygen sensor incorrecly diagnosed

I had the check engine light diagnosed at Autozone, was told it was the FRONT O2 sensor. Friend replaced it - light went out. Few hours later when I started the car again, light came on. Took car to mechanic who said it was REAR sensor. Now I’m out $150 for replacing good part and can’t afford to replace bad one. How long can I drive like this? Car is 2004 w/57K miles.

When one goes, the others can’t be far behind…Cars don’t wear out one part at a time…The entire car wears out as a unit…If you can’t afford to maintain a car you should not own one. You can drive “like this” until your next emissions test…

Of the two sensors, the front is the most important as it controls the fuel mixture delivered to the engine. The rear one just monitors the condition of the catalytic converter…They work as a pair and they should be replaced as a pair…

Don’t be completely certain that a mistake was made. On an OBD II vehicle, the two sensors work together and the most common failure mode is that they still put out the appropriate voltage but the response time gets slow. Usually, the front sensor fails before the rear. Once a fresh (quick) sensor was paired with an old (slower) sensor, it is not surprising that the computer set another code.

Back in the OBD I days, the system was much more forgiving of an old slow oxygen sensor, so we ran oxygen sensors 200k miles, if the fuel was good and the engine was running well. OBD II systems are more particular.

I would speculate that you may well have been due for both sensors, though 57k miles is quite young for failure. Is there any chance they were damaged by gasoline of questionable source, or additives or sealants used in the engine?

AutoZone does not diagnose problems nor should they. They will scan the car and provide a code. This code is nothing more than a starting point for diagnosis.

Maybe they provided you with a code that points to the front O2 and you assumed based on that they were diagnosing the problem as such.

Could the DTC code pulled by Autozone been related to the O2 sensor heater circuit? If you have the code(s) actually pulled, please include those as it helps us provide the appropriate advice. Did the second mechanic pull a DTC code? Was it different from the previous code(s) pulled.

Hope to help.

I don’t appreciate the snide remark “if you can’t afford to maintain a car you shouldn’t own one” I’M UNEMPLOYED!!! Hope you’re never in the position of not being able to afford what you need.

Thanks for the helpful info - I was surprised that the sensor went as I’ve had cars up to 92K miles and never had a sensor problem. No additives or sealants have been used in the 4 years I’ve owned the car and I fill up at 2 different gas stations (both brand names) - whichever is cheaper.

AutoZone says they run diagnostics for free - I went there on the recommendation of a friend who buys his parts there. The guy said “your front sensor is out” so I believed him. I’ve learned a lesson the hard way. Thanks for your response.

If you purchase auto parts online in some cases you can save big bucks. You could have purchase both sensors for less then you paid for the one. Here is a link to one such online source http://www.rockauto.com/catalog/x,carcode,1425619,parttype,5132

Please see my reply to the previous response. I don’t know what the codes are (wish I did - I’ll know for next time) - thanks for that tip!!
What I do know is that the AutoZone diagnostics “tool” was about the size of a cell phone and it took the guy about 2 minutes to give me an answer.
The AAA approved mechanic had a “computer” about the size of an I-pad and interacted with it for about 5 minutes before giving me an answer.

Thanks for the link - I first bought the part from AutoZone, but the connector didn’t fit - I called around to other auto parts stores and a Hyundai dealer and learned that the part had been changed, but not all computers are up to date(???) A very nice guy at Napa made several calls for me, including to Hyundai direct and ordered the right part at a price better than anyone else.

There Exist More Than 3 Dozen OBD2 Codes Diectly Related To 02 Sensors And Hundreds Of Others That Could Implicate Them. Wow, Those Lowly Little Plugs Get Blamed For Everything !


This is the first time I’m hearing something like that from all my conversations on-line and off. SOOOOO, the question is how does one really know when something is accurately diagnosed? Whom can you trust to be knowledgeable AND reliable??

The Autozone guy did NOT run diagnostics on your car with his “cell phone sized” device. Your car has it’s own computer, and the device the Autozone guy used simply read the diagnostic codes that your car’s computer stored.

Diagnostic codes are a place to start troubleshooting, they inform the reader what the computer perceives as a problem. The car’s computer is not, as Hollywood would have you believe, all-knowing and all-seeing. Your car may very well have had more than one problem, or the code may have meant that the oxygen sensor CIRCUIT wasn’t working, but that doesn’t mean that the oxygen sensor itself is to blame. Or the Autozone guy may have simply interpreted the code wrong. The possibilites are endless. As I’ve said before, the code is merely a place to start troubleshooting.

OK, so what is the next step in the troubleshooting process? With endless possibilities how does one know when something is accurately diagnosed? I’m really trying to learn how be smart about this.

In my experience at least 90% of the time when a car brings up a O2 sensor code replacing the sensor fixes it. However there is always a possibility that its something other then the sensor, a wire connection or ECM problem for example. But we need to consider about labor rates of up to $100.00 a hour to run a costly diagnostics just to find out it is the O2 sensor.

In the case of a O2 sensor the least expensive diagnostic is to replace the sensor with a new one to see if it resolves the problem, because the diagnostics and cost of the equipment to truly without question determine its is the sensor can exceed the cost of the part.

I am in no way advocating being a parts changer, however when it come to the O2 sensor the odds are on your side.

Sometimes the computer throws a code that translates “Catalytic converter efficiency below threshold”…Many people rush out and replace the converter when all it REALLY needed was the aft oxygen sensor, the part that caused the converter code to pop up…

One thing I do before replacing a O2 sensor is inspect the connector at the sensor, unplug it inspect the pines for corrosion then re-connect the connector, and clear the code. If the code does not come back then it was the connector, and you saved the cost of a new sensor.

                            Good Luck

THIS COMMENT IS SPECIFICALLY DIRECTED TO “OLD SCHOOL” – oh please, I never said my skills are in automechanics! You would think of making some money doing what I do best - could you be any more condescending? Do you watch the news? Do you know what the employment situation is? I’M UNEMPLOYED LIKE MILLIONS OF OTHERS!!! Sarcasm is not helpful here. I hope you never know what it’s like not to have enough money for what you need.

Thanks for one of the most practical suggestions.