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How to Check an Oxygen Sensor

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Checking an old oxygen sensor.

Checking an old oxygen sensor.

Learn How to Check an Oxygen Sensor

Learning how to check an oxygen sensor (O2) will help you confirm a potential problem with the sensor in your vehicle before you spend time and money replacing a unit that might not even need a replacement in the first place. Oxygen sensors detect the amount of oxygen content in the exhaust stream and turn this information into a voltage signal that your car computer uses to control fuel ratio and emissions.

However, whenever the sensor’s signal changes—and stays out of—its normal operating parameters, it causes the computer to store a trouble code in memory and turn a malfunction indicator light (MIL) or Check Engine light on your dashboard to alert you of the problem.

Still, whether you suspect a bad oxygen sensor or a computer trouble code points to a potential issue with the O2 sensor, the problem may lay somewhere else. All the computer does is inform you where the problem was detected. For example, you may have a loose or torn vacuum hose, causing the sensor to read a constant high rate of oxygen in the exhaust stream; or the sensor electrical connector may have become loose, preventing the device from working. Instead, the computer detects a sensor abnormal operation.

So before replacing the unit, you can use the sensor’s operating characteristics to verify whether you actually need to replace it.

An oxygen sensor may be located near the exhaust manifold.

An oxygen sensor may be located near the exhaust manifold.

Connecting Your Voltmeter to the O2 Sensor

To run this test, you will need a 10-megaohm impedance digital voltmeter. Most digital voltmeters come with a 10-megaohm protection to prevent the meter from drawing too much electrical current and damaging electrical or electronic components during a test.

Also, before you start your tests, locate the oxygen sensor you want to troubleshoot. On pre-1996 vehicle models, you'll find the sensor usually on or near the exhaust manifold. On 1996 and newer models, you'll see a sensor near the exhaust manifold, and another one near the catalytic converter. However, some vehicle models have up to five or more sensors. Make sure you know which sensor you need to check.

When you retrieve diagnostic trouble codes (DTC) from your car computer, you may also get information about the specific sensor at fault, depending on your scan tool features. For example, you may get a Bank I, sensor 1 at fault, which points to the O2 sensor on or near the exhaust manifold on the cylinder head that contains cylinder number 1. A bank I, sensor 2, points to the sensor on the same side but farther down the exhaust system, probably right before or after the catalytic converter. The same goes for the other cylinder head—on V-type engines—which is considered Bank II.

To locate Bank I and Bank II, consult your vehicle service manual, if necessary.

Then, if the sensor you're testing has more than one wire (heated sensor), locate the signal wire by consulting your vehicle service manual, if necessary.

  • A single-wire sensor uses that wire as the signal wire.
  • Two-wire sensors use one wire for the sensor's signal and the other one to power the heater.
  • Three-wire models, use one wire for the signal and the other two wires to power and ground the heater.
  • Four-wire sensors, though, use one of the wires to ground the sensor itself.

To identify the wires look at the wiring diagram in your vehicle repair manual. If you don't have the manual, buy an inexpensive, aftermarket manual for your specific vehicle make and model at your local auto parts store or online.

  1. Once you have the appropriate voltmeter on hand and have located the sensor, warm up your car engine to operating temperature. You can do this by taking your car for a 20-minute drive on the highway or idling the engine for about 15 to 20 minutes at a fast idle speed.
  2. Turn off the engine and set your voltmeter to the mV (millivolt) DC scale.
  3. If you are testing an O2 sensor near the catalytic converter, lift your vehicle using a floor jack and safely support the vehicle on a couple of jack stands and block the rear wheels.
  4. Be careful when connecting your meter. When the engine is at operating temperature, the exhaust manifold and pipes are extremely hot. Don't burn yourself and keep your meter and probes away from hot surfaces.
  5. On sensors with one to three wires, connect the meter’s red probe to the sensor's signal wire and the meter’s black probe to a good ground on your engine. On four-wire sensors, connect the meter's black probe to the sensor's ground wire. If necessary, consult the wiring diagram in your vehicle repair manual.

To connect your meter's probe to the wire, use a wire-piercing probe or back probe the sensor through the connector. With some sensors, though, it's difficult to back probe the signal wire through the connector. To overcome this limitation, you can unplug the sensor and connect a strand of copper wire to the connector prong for the signal wire, and plug back in the electrical connector, leaving the stretch of wire sticking out of the connector. This will give you a bare wire you can connect to your meter probe for the test. Just make sure the bare wire doesn't touch ground.

Another option is to pierce the sensor's signal wire through the insulation with a pin and connect the meter probe to the pin. But keep the pin from touching ground.

If you decide to go with the latter method, after you finish your tests remove the pin and cover the pierced section of wire with electrical tape to prevent moisture and corrosion from creeping into the wire.

The Oxygen Sensor Helps Reduce:

  • Hydrocarbon
  • Carbon monoxide
  • Oxides of nitrogen

Reading the Oxygen Sensor’s Signals

Start the engine and check the sensor’s voltage signals on your voltmeter. The sensor voltage should cycle or fluctuate within the 100 mV-900 mV (0.10 to 0.90V) range approximately. This means the sensor is operating properly.

If the O2 sensor only produces a low- or high-voltage signal, either you have an engine performance issue or the oxygen sensor stopped working. To verify sensor operation, conduct the next two tests.

Test the Oxygen Sensor Response to a Lean Fuel Condition

  1. First, disconnect the hose from the positive crankcase ventilation (PCV) valve leading to the intake manifold. This will allow more air to enter the engine. If you need to locate the PVC valve, consult your vehicle service manual.
  2. Check the sensor’s signal voltmeter reading. An oxygen sensor interprets an increase in oxygen as a fuel lean condition and emits a signal close to 200mV (0.20 V). If the sensor doesn't respond accordingly or takes time to respond, the sensor isn't working properly.
  3. Reconnect the hose to the PCV valve.

Test the Oxygen Sensor Response to a Rich Fuel Condition

  1. Disconnect the plastic duct from the air cleaner assembly on your vehicle.
  2. Block the duct opening leading to the engine with a clean rag. This will reduce the amount of air going into the engine.
  3. Check the sensor’s signal voltmeter reading. An oxygen sensor interprets a decrease in oxygen as a rich fuel condition and emits a signal close to 800mV (0.80 V). If the sensor doesn't respond accordingly or takes time to respond, the sensor isn't working properly.
  4. Reconnect the air duct to the air cleaner assembly and turn off the engine.

If the oxygen sensor in your vehicle responded correctly to your tests, you might have a problem with another component affecting fuel efficiency. The engine may have a vacuum leak, a problem in the ignition system or something similar. If your sensor didn’t respond properly to your tests, the sensor has stopped working and you need to replace it.

Digital Multimeters

Replace the oxygen sensor at the recommended manufacturer interval for better engine performance.

Replace the oxygen sensor at the recommended manufacturer interval for better engine performance.

Replace the O2 Sensor When Needed

Once you know how to check an oxygen sensor, you can verify whether the unit is actually working or in need of replacement. These tests will save you money and time—and help you fix your vehicle sooner. Also, you might want to check the service interval for your O2 sensor in your repair manual. As driving miles accumulate, exhaust byproducts cover the tip of the unit that protects the sensing element. Then the sensor efficiency drops, causing engine performance issues. Eventually, the sensor stops working. So it's a good idea to replace it at the recommended schedule to restore fuel efficiency and reduce emissions.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: Can a bad O2 cause a P0430 code?

Answer: It is possible. The oxygen sensor located downstream from the catalytic converter is usually the one that makes the ECU throw this code. This O2 is a cat monitor, and when the computer senses the catalytic is not doing its job based on the feedback from this sensor, it lights the CEL. However, other faults can make this code appear, like an exhaust leak, a bad O2, a misfire or a leaking fuel injector.

Question: What is the stationary percentage setting for the 02 sensor?

Answer: The average for that sensor is 0.5. It constantly fluctuates between 0.9 and 0.1, approximately.

Question: Why would my engine RPM increase when I pump my break peddle?

Answer: The master cylinder might be leaking and fluid entering the brake booster. Applying the brakes can send the fluid into the engine, having that extra boos effect in rpms.

Question: My rear o2 sensor is 2 fixed in 0.7 voltage. Is this normal?

Answer: The rear sensor monitors the catalityc converter job. The downstream O2 sensor fluctuates very little if the cat is working properly. Voltage reading probably an average of 0.5 volts and sometimes higher.

Question: I've checked my O2 sensor output voltages using an OBD scanner. The downstream sensor fluctuates between 0.1 and 0.9 V. The upstream sensor isn't giving me any output, just a dash on the scanner tool display. My LTFT hovers around 35% on idle and drops close to zero at load. Any idea what's going on?

Answer: Seems like there could be a vacuum leak. This may include a leaking EVAP purge valve and crankcase ventilation system.

Question: Can I have the Ohms value for these sensors?

Answer: Look up the specs in the repair manual for your particular vehicle make and model. If you don’t have the manual, check the reference section of your local public library for it. Identify the heater terminals and check resistance with your multimeter. Compare your readings with those listed in your manual. If the values don’t match, replace the sensor.

Question: If one of the four oxygen sensors is bad, will that cause the bad sensor to throw codes for all of them? Or does multiple oxygen sensor malfunction sound more like a ground wire issue?

Answer: This would be more like a ground wire problem.

Question: What if the O2 sensor bends are not?

Answer: Use a sensor adapter. As long as the head of the O2 remains in the stream, it'll work.

Question: The front sensor on my 07 Subaru Outback hangs out around 350mV at idle and doesn’t seem to fluctuate from there. Blocking the air intake to the filter box didn’t do anything, and disconnecting the hose from the PCV valve caused the engine to rev in waves til it eventually stalled. Does the fact that the sensor didn’t fluctuate indicate an issue with it?

Answer: The front sensor should be constantly fluctuating between (near) 0.1 and 0.9 volts.

Question: My 8.1 dies going down the highway. I turn the key off, it starts right up and runs great for a while before quitting. What could be the problem?

Answer: This other post might help you:

Question: Is it possible for an o2 sensor to go bad without throwing a CEL?

Answer: If the sensor is worn but still within the working parameters, it won't trigger a trouble code.

Question: My O2 sensor is reading 0.4xx no matter what the conditions are (idle or load). LTFT is also high (+32% idle and +12% load). There aren't any error codes. The engine is a 1.6 16V AUS (VW). The O2 sensor is also new/ Does this mean a bad signal or a ground wire to O2?

Answer: It seems like a vacuum leak. Check for leaks using carburetor cleaner. They are hard to find sometimes. A smoke test, if necessary. Other potential problems: a bad IAC (idle air control) valve, EVAP solenoid, EGR valve.

Question: Can I test the voltage on a downstream air sensor in the exhaust while unplugged? Like pulling it out and using a torch idea?

Answer: Yes. The heat from the propane torch eliminates the oxygen from around the sensor, which causes the sensor to send a voltage signal.

Question: On my scanner, the oxygen sensor reads a fixed 1.5 volts and 10% lean condition. Can a sensor reach 1.5 volts? Is the sensor broken?

Answer: A titania oxygen sensor can read up to 1.5 volts, and they read in the opposite direction (low voltage will point to a rich condition while a high voltage will indicate a lean condition – others fluctuate between 0 to 5 volts. Check the type of system your vehicle uses first and operating parameters. You may find this information in your vehicle repair manual. It may also tell you how to test the sensor.

Question: How can I get the OEM original sensor?

Answer: Check with your local dealer.

Question: Every source says to "backprobe" an O2 sensor. The O2 sensor is a bimetallic junction that produces a voltage when heat is applied. And I did just measure a voltage with the sensor disconnected from the harness and measuring only the sensor's signal wire and ground, engine running. So, is there a valid test protocol doing it this way?

Answer: Actually, the recommended tests make use of a scope or scan tool, while monitoring sensor switching response time while idling, and load conditions. I haven't heard of a specific protocol to use while the sensor is unplugged though.

Question: One of the cables broke on my oxygen sensor. Is it possible to fix seeing as though it is connected to the exhaust manifold and gets so hot? At the break of the cable is right at the sensor.

Answer: It might be possible but I think you'd be better off replacing the sensor.

Question: How does the front 5 wire oxygen sensor work?

Answer: That's commonly referred to as an air/fuel ratio (wide band) oxygen sensor. These newer sensors output a 0-5 volt signal to the engine control unit (ECU). This video here gives you a good explanation of how it works:

Question: My check engine light is on for my Nissan Maxima 2000. I've been told it's both downstream sensors, which I'll try to replace myself because they're charging a bit. Is the reflashing necessary or should I do that first to see if it's even the sensors? I am trying to hurry for inspection. Oh, and what tools would you recommend?

Answer: You can double check by going to one of your local auto parts stores and have them download the trouble codes for you. If you want to replace them yourself, use an oxygen sensor socket. Check them online to see which one will adapt better for your needs (check the position of the sensors in your vehicle). You may need an extension to use with your ratchet. This other post may help:

Clear the codes after replacing the sensors.

Question: Can a bad 02 cause a P2195 code?

Answer: There could be several reasons for this code to appear: fuel injector issues, MAF sensor problems, low fuel system pressure, vacuum leaks, a leaking PCV system, and of course a faulty O2 sensor or circuit.

Question: Where are the 02 sensors on my car?

Answer: Look down the exhaust header pipe(s), On modern cars, there's one right before the catalytic converter and one after. Older vehicles used only one before the cat.

Question: My 2010 VW Tiguan has a high gas consumption. I checked the live data with my scanner and I have STFT -8 and LTFT -3. My O2 sensor shows me 0.780 mv, which means Rich condition. Could my car's high gas consumption be because my O2 is bad? I don’t have a check engine light either.

Answer: Make sure you don't actually have a rich condition, there could be a leaking fuel injector or some other issue. Maybe this other post can help: