Faulty Oxygen Sensor Symptoms & Diagnosis

Knowing when your car’s oxygen sensor has failed can save hundreds of dollars in diagnostic fees and wasted parts. The oxygen sensors are a vital part of the engine management system and are used to perform two very important functions.

Catalyst performance

The bank 1 O2 sensor is typically mounted in your vehicle’s exhaust manifold and monitors the air/fuel mixture (ratio of parts air and parts fuel being consumed), while the downstream or bank 2 O2 sensors are used to monitor the performance of your vehicle’s catalyst.


Most O2 sensors generate a rich or lean voltage signal depending in on how much unburned oxygen is metered to be flowing through the vehicle’s exhaust, usually mounted near the collector on the manifold where exhaust from each combustion chamber meets.  A few, like the heated ceramic O2 sensors in some Nissans, older Jeeps, and Eagle, vary in resistance to indicate a rich or lean condition in the exhaust, instead of the traditional voltage signal, but these are far less common.

The most common of symptoms related to a faulty oxygen sensor is a sudden (significant and almost immediately noticeable) drop in your vehicles fuel economy. Another immediate effect of a defective sensor is a drastic change engine performance, overall power and response.  But there are other tells that could point to a faulty sensor, for example your car could idle significantly harsher then normal, or sporadically fluctuate idle. Also an odor from your exhaust from of the extra fuel being consumed is possible as well.

Fuel management

Generally, for fuel management purposes, your engine’s ECU would use the input from the bank 1 O2 sensor to readjust the fuel mixture as needed for optimum emissions, fuel economy and performance. This is why a common complaint for below average fuel economy, weaker than normal engine responsiveness, and performance is routed to a faulty front O2. Signal for the second bank of oxygen sensors is used primarily to detect any problems with the vehicles catalyst, and to tune in fuel trim. Many late model vehicle are replacing the traditional bank 1 sensor for an air/fuel sensor (which is just another name for a wide-band oxygen sensor) to monitor the fuel mixture.

The wide operating range of wide-band O2 sensors allows for faster reaction to correcting fuel trim and keeps the engine’s operating range as close to stoichiometry (14.7:1 air fuel ratio), as possible. In doing so this keeps newer engines capable of reducing emissions by a great deal, keeping engines more compliant to the stricter emissions laws of today. But these systems also make these sensors far more expensive than the average automotive oxygen sensor.

Now the OBD 2 system should detect if any faults with your vehicle’s oxygen sensors are present, whether there are related to the internal heater or the circuit leading to your sensor and set one or more fault codes turning on the check engine light. If diagnosis confirms a faulty sensor (and not a wiring issue or alternate engine fault that could cause a similar situation), the sensor should be replaced.

It has been my experience that vehicles with multiple bank sensors, where a bank 1 sensor failed, the other bank 1 sensor on the opposite bank will likely need replacing as well (I feel as if I have yet to own a car that did not almost immediately need a matching bank sensor when replacing the originating fault sensor). Though some front sensors can be costly, replacing both at the same time ensures you get the best possible performance and fuel economy, given the importance to fuel mixture.

02 diagram

The bank 2 oxygen sensors tend to outlast front O2’s as they are behind the catalyst and are exposed for far less heat than normal. However they are also exposed to more road debris given their positioning and open space. Though most people rather not, I usually recommend replacing any bank 2 oxygen sensors when a catalyst is replaced as to better ensure the life of a new catalyst, but also to adhere to the policies of some catalytic converter manufacturer warranties.

Now you can get oxygen sensors in two variants; exact fit units and universal sensors. Exact fit O2 sensors come with the complete connection harness and are “plug and play” made per application. Universal units are simply pig tail sensors that would require you to cut and splice them into your vehicles original sensor using that connector to meet to your existing harness, but allow suppliers to offer fewer part numbers that cover larger ranges of applications.

Some people are hesitant to using the universal type sensor given the work entailed to install them, but for vehicles with the harder to source sensors or stranger engine combinations they are better for those with non US applications. A universal sensor was the only option I had for a Nissan I had previously where the engine was from a vehicle not offered here in the states. Splicing was easy and operation was just as good as the original when it was in working order. Also offer a more economical alternative to direct fit units.

Shop Oxygen Sensors at FCP Euro

About The Author: Mike Rivera

mikerMike Rivera is an ASE Certified Technician and Senior Sales Account Executive at FCP Euro. He can be reached at mike@fcpeuro.com


About these ads

Categories: Advice, General Advice

20 replies

  1. How does one diagnose a “lazy” O2 sensor? What is considered lazy. I know one would need a real-time OBD scanner and watch the O2 sensor fluctuate voltage but in general what should this fluctuation rate be? The once in a while rotten egg odor, is that telling you the O2 is getting ready to fail?

  2. One thing he doesn’t mention is the need to troubleshoot why a Bank 1 sensor would fail. Bank 1 sensors may fail because the sensor malfunctions. Or, a Bank 1 sensor might fail if there’s a fuel trim problem upstream. Replacing the sensor without checking out these possibilities could amount to sacrificing a new sensor because the original problem has not been addressed. Same is true of the Bank 2 sensor. If the O2 sensor behind the catalyst fails, it may mean the catalyst is no longer working because of road damage or contamination from a chemical (such as a coolant leak) that corrupts the metals in the catalyst. There are ways to troubleshoot whether the catalytic converter is working and this is one of the steps you may need to work through before buying new O2 sensors. Just my two-cents worth.

    • I was wondering the same thing about the bank 2 sensors: how would a person know if the error code means the sensor is bad or if the catalytic converter is bad?

      • If your cat is bad and you’re lucky enough to have a scanner that can graph them, then the front and rear sensors will mirror each other pretty much perfectly. The other way is to take a temperature reading (most convenient way is with a laser temperature device) before and after the cat. If the temperatures are nearly the same, the cat is most likely bad. If behind the cat is significantly hotter than the front, then the cat is most likely doing its job. Remember to make sure the car is nice and hot before testing as the sensor will be using default parameters until it reaches optimal temperatures.

  3. Most O2 sensors are diagnosed as bad because of another problem. It is important that you test your sensor before you incorrectly condemn it.

    Testing O2 sensors on the workbench.

    Use a high impedance DC voltmeter. Clamp the sensor in a vice, or use a plier or vice-grip to hold it. Clamp your negative voltmeter lead to the case, and the positive to the output wire. Use a propane torch set to high and the inner blue flame tip to heat the fluted or perforated area of the sensor. It will glow red hot, and this process will burn some if not all of the carbon fouling off of it. You should see a DC voltage of at least 0.6 within 20 seconds. If not, most likely cause is open circuit internally or lead fouling. If OK so far, remove from flame. You should see a drop to under 0.1 volt within 4 seconds. If not likely silicone fouled. If still OK, heat for two full minutes and watch for drops in voltage. Sometimes, the internal connections will open up under heat. This is the same a loose wire and is a failure. If the sensor is OK at this point, and will switch from high to low quickly as you move the flame, the sensor is good. Bear in mind that good or bad is relative, with port fuel injection needing faster information than carbureted systems.

    ANY O2 sensor that will generate 0.9 volts or more when heated, show 0.1 volts or less within one second of flame removal, AND pass the two minute heat test is good regardless of age. When replacing a sensor, don’t miss the opportunity to use the test above on the replacement. This will calibrate your evaluation skills and save you money in the future. There is almost always *no* benefit in replacing an oxygen sensor that will pass the test in the first line of this paragraph.

    • I think you answered my question above but I can’t seem to delete that now that it’s posted.

      If I get an error code that says I have a failed bank 2 sensor I’d have to test it to confirm it’s bad. If it’s confirmed as good then I’m looking at replacing the catalytic converter. Thanks for the detailed procedure above!

  4. I like Mike’s use of the term “stoichiometry” in his article. Fancy word – haven’t seen or heard that since college. Awesome!

  5. People should not forget the issues brought upon by a biased sensor. Where it wont go quite high enough or quite low enough in voltage properly making the switch point (the average of the rich/lean voltage) biased either rich or lean. As a smog tech ive seen NoX limits get lower and lower. Realistically it is not an issue if everything is working. But as things age and systems become partially clogged or dirty its important to be sure everything is in order. A biased sensor may not seem like it might be such an issue, but a sensor biased rich will drive a system lean, and even just a little in cars without a nox control device like EGR, that can really send you through a loop. Ive had more than one nissan altima with nox issues. Aside from the egr passages cloging in the intakes, and leaky intake gaskets causing the motors to run lean, ive had aftermarket sensors, brand new be biased, pushing it slightly lean and raising nox.

    Dont buy cheep sensors, you will be doing nothing but replacing a bad part with a questionable one.

  6. OK, a few points to start. It might be interesting to know where that O2 sensor bench test came from, however the response time of a good O2 sensor (in place) is less than 100 milliseconds and of course measured with a DSO and a procedure to drive the engine rich and lean in a timely fashion. This is a proven and accepted repair industry tactic from several years ago. Learning more about the sensors and actuators and their roles in modern engine management is critical if you want to make informed choices and purchases. Let’s move on to properly identifying the O2 sensors as to position and role. Previous to the US 1996 model year we found only one O2 sensor on an inline engine or possibly two on a vee configuration. Back then as it is still the case today the O2 sensor that is first in line (or upstream of the catalytic converter) is responsible for fuel trim. This O2 (sensor 1) constantly relays the free or unbound oxygen content of the exhaust stream back to the PCM which adjusts the fuel mixture about that ideal (stiochiometric point) air /fuel ratio of 14.7 units of air to 1 unit of fuel. On a vee engine two of these sensors would be identified as Bank 1 Sensor 1 and Bank 2 Sensor 1. Along comes model year 1996 in the US and all light vehicles must conform to OBD 2, an emissions mandate that has the vehicle’s PCM report on the vehicle’s ability to run cleanly. The downstream (Sensor 2) O2 sensor is in place for only one primary reason: to report on the efficiency of the catalytic converter that’s blowing in its face. Sensor 2 has no bearing on the life of the converter other than to condemn it if the converter cannot retain enough free oxygen. This trailing O2 sensor is only used by the PCM when conditions are appropriate, such a steady cruise with engine up to temperature, etc. Learn to use a scan tool and read its parameters for your car when its running well.
    Any questions? ………………….just sayin’

  7. Regarding the save vs replace discussion, having done sensors on both an Audi 4000Q and Volvo 850T and I opted for the universal replacement Bosch which worked fine in both cases. I didn’t bother to even test since the level of effort of removing/installing the sensor far outweighted the $50 or so for a part which will likely expire at some point. In the 4000Q I had to remove the cat and extricate the sensor on a bench (uh needless to say, it was “damaged” in the process). On the 850T, the hardest part was figuring out how the OEM connector worked! The 850Ts have two O2 sensors and my experience is that the rear that usually goes first (fault codes do say which). For me, the front acted up about 100K miles after I did the rear so don’t assume its a replace both kind of job.

  8. Bank 1 is not upstream and Bank 2 is not downstream. On vehicles with a single exhaust upstream sensors are usually designated Bank 1 Sensor 1 and downstream as Bank 1 Sensor 2. Vehicles with a two exhaust systems will add Bank 2 which may be left and right halves of a V engine or front and back halves of an inline 6. People who write articles should know this.

    • Yep , TheDurkee is right !
      Bank 1 is not the same as upstream and Bank 2 in not downstream sensors.
      Unfortunately that very basic error has been promoted thru the whole article.
      My advice is go shop somewhere else.

  9. Please help. I have a Kia Cerato ex 2008. My engine had an overall. Received it back the oil light kept comming on if the reffs are under 1000. Took my car back to machanic that is also a auto electrician. He send the car tho the engineers that did the head. The engi engineers told me thatthe send the car to kia before and kia told themthe oxygen sensor is faulty. The told the auto electrician to fix it first before they rebuild the engine again on the guarentee. Needless to say the auto electrician did not inform me or replaced it. The engineers rebuild the engine. My car back with the same problem! I called both auto electrician and engineer. Then it was apperently loose wire but no change in my car! Then the engineer told me about the faulty oxygen sensor. I took my car to Kia to be tested. On my way there I had to keep the reffs at 2000 so that the car would not die. Kia informed me of the oxygen sensor as well as that my car overheated(thatt did not show) head needs overall again for the 3rd time, the radiator also has a crack in now!. What I want to know now is who is the one in fault. Could it be because the oxygen sensor was not ?replaced?

  10. Help please.
    Subaru Impreza R 2009. 12 months ago, 3 fault lights came on, and mechanic replaced Bank 1 Sensor 1 as “heated sensor has shorted out”. A day later lights reappeared, and Bank 1 Sensor 2 “voltage too low” and was replaced, which is consistent with the author’s experience.
    About 5000kms later, same fault lights came on again, and dIagnosis was “catalyst system below threshold” and “Bank 1 Sensor 1 voltage was too high 2.184V”. This time, mechanic just cleared fault code.
    Another 5000kms driven and same fault lights appear, and diagnosis was “Bank 1 Sensor 1 voltage was too high”. However mechanic also said “Live data on scan tool showing oxygen sensors working correctly” so he just cleared code. He did test battery, but “battery and charging system tested ok”.
    Can anybody suggest why this keeps happening, please?
    Also, will it damage my car to not determine the root cause?

    • Be 100% sure they replaced with the Subaru part number for the O2 sensors. At this point and the fact it isn’t an old car, I would bring it to a Subaru dealer. Yeah, you might get ripped off, I’m betting they’ll say something about replacing the CAT but something isn’t right. First guess is your mechanic used an aftermarket O2 sensor and if Subaru is like Volvo, that is a bad thing to do. O2 Sensors, spark plugs sometimes you are just better off sticking with what the engine was built with.

  11. Thanks so much Rich – I only just saw your reply!
    Light has just come on again. I know for a fact that the replacement of Bank 1 Sensor 1 was not a genuine part (without consulting me – another story!) so I am going to demand that the mechanic replace it with a genuine part – at no cost to me.
    Thanks again

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s