Knowing when your car's oxygen sensor or O2 sensor (sometimes referred to as 02 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. They are used to perform two critical functions—plus, they just so happen to be easy to replace.
Symptoms of a Failing Oxygen (O2) Sensor
The most common symptom related to a faulty oxygen sensor is a sudden (significant and almost immediately noticeable) drop in your vehicle's fuel economy. Other immediately apparent effects of a defective sensor is a drastic change in engine performance, overall power, and response. But, other symptoms that could point to a faulty sensor, for example, your car could idle significantly harsher than normal, or sporadically fluctuate when idling. An odor from your exhaust due to the extra fuel being consumed is possible as well.
- Poor fuel economy
- Poor engine performance
- Rough idle
- Rich fuel smell from exhaust system
How Oxygen Sensors Measure Catalyst Performance
The upstream 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 O2 sensors are used to monitor the performance of your vehicle's catalyst.
Most O2 sensors generate a rich or lean voltage signal depending on how much unburned oxygen is measured to be flowing through the vehicle's exhaust, and is typically mounted near the collector on the manifold where exhaust from each combustion chamber meets. A few sensors, like the heated ceramic units in some Nissans and older jeeps, vary in resistance to indicate a rich or lean condition in the exhaust, instead of the traditional voltage signal. However, these are far less common than the former.
How Oxygen Sensors are Used for Fuel Management
Generally, for fuel management purposes, your engine's ECU uses 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 when why people complain of below-average fuel economy, weaker than normal engine responsiveness, and overall performance, all of which point to a faulty front O2 sensor. The signal from the second bank of oxygen sensors is used primarily to detect any problems with the vehicle's catalyst and to tune the fuel trim. Many late-model vehicles 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 operating range of wide-band O2 sensors allows for faster reaction time when 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, reducing emissions on a newer vehicle by a great deal to comply with the stricter emissions laws of today. Because of this, however, these wide-band O2 sensors are much more expensive than the traditional O2 sensors.
Now, the OBD II system on all modern cars like the Volvo XC90 and BMW X5 should detect if there are any faults with your car's oxygen sensors, whether they're related to the internal heater or the circuit leading to your sensor, and give out one or more fault codes turning on the check engine light. If the diagnosis confirms a faulty sensor (and not a wiring issue or other engine fault that could cause a similar situation), the sensor should be replaced ASAP.
In my experience, vehicles with multiple bank sensors that had a faulty bank 1 sensor, the other bank 1 sensor on the opposite bank will need replacing soon. 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 air/fuel mixture.
The bank 2 oxygen sensors tend to outlast the fronts as they're behind the catalyst and are exposed for far less heat than the bank 1 sensors. However, they're also exposed to more road debris, given their positioning and open space. Though most people would rather not, I usually recommend replacing any bank 2 oxygen sensors when a catalyst is replaced to increase the lifespan of the catalyst and to adhere to the policies of some catalytic converter manufacturer warranties.
Types of Oxygen Sensors
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 for a specific application. Universal units are simply pigtail sensors that would require you to cut and splice them into your vehicle's original sensor using that connector to meet your existing harness. This allows suppliers to offer fewer part numbers that cover larger ranges of vehicles and applications.
Some people are hesitant to use the universal type sensor given the necessary work to install them. They're more suited for vehicles with the more difficult to source sensors, strange engine combinations (think engine swaps), or for applications outside of the USA. Just as an example, my only option was a universal sensor for a Nissan I had previously, where the engine was from a vehicle not offered here in the states. Splicing was simple, and in operation, it was just as good as the original. Another added benefit of universal O2 sensors is that they are more cost-effective that exact fit units.
How to Test Oxygen Sensors
Editors Note: "KG" in the comments below added this incredibly helpful short instruction on how to test your sensors.
"For this, you're going to want to use a high impedance DC voltmeter. Start by clamping the sensor in a vice, or use pliers or Vice-Grips to hold it. Clamp your negative voltmeter lead to the case and the positive to the output wire. With a propane torch set to high, use 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 twenty seconds. If not, the most likely cause is an open circuit internally or lead fouling. If everything is OK so far, remove from flame. You should see a drop to under 0.1 volts within four seconds. If not, it's likely silicone fouled. If it's still OK, heat the O2 sensor for two full minutes and watch for drops in voltage. It can be that sometimes the internal connections will open when heated. If the sensor is still 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 shows 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's benefit in replacing an oxygen sensor that will pass the test in the first line of this paragraph."
If you read this far, you probably have a feeling your oxygen sensors are faulty. And if that's the case and you're wondering which ones you need for your car, you can go to our home page and use our vehicle selection tool in the upper-left corner. This will ensure you buy the exact sensors that fit your vehicle. And if you didn't know, these O2 sensors (which can be pricey), like everything else we sell, are covered under our Lifetime Replacement Guarantee. If you have any issues with them down the line, just send them back to us for a brand new replacement.
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Writer/Editor at FCP Euro and owner of a daily R53 MINI Cooper, a track-built R53 MINI, and a 1997 Dakar Yellow E36 M3 Sedan. ••• Instagram: @evan.madore