How to Check an Oxygen Sensor

Checking an old oxygen sensor.
Checking an old oxygen sensor. | Source

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 you 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. | Source

Always use a 10-megaohm impedance digital multimeter to prevent damage when working with sensitive electronic and electrical devices on your vehicle.

Connecting Your Voltmeter to the Oxygen 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 5 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, locate the signal wire by consulting your vehicle service manual, if necessary. If you don't have this manual on hand, you can consult one by going to the reference section of your local public library; or better yet, 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. Connect the meter’s red probe to the sensor's single electrical signal wire (pre-1996 and some newer models) and the meter’s black probe to a good ground on your engine (some of these sensors may have two wires, though). If your vehicle uses a heated oxygen sensor (1996 and some newer models), make sure you connect to the sensor's signal wire. This sensor may have two to four wires.

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:
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. Next, 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

INNOVA 3320 Auto-Ranging Digital Multimeter
INNOVA 3320 Auto-Ranging Digital Multimeter

Home and Automotive Use

The INNOVA 3320 is a Auto-Ranging Multimeter designed to safely and accurately troubleshoot a variety of automotive and household electrical problems.

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. | Source

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.

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