Understanding oxygen sensors: what they are and what they do

Emission control systems play an important role in our cars—making them not only better for the environment, but easier on your wallet by improving fuel efficiency. In petrol engines, this system is the humble oxygen sensor. A faulty or lazy oxygen sensor can impact how your car runs and performs, so if you’ve noticed something’s off—your O2 sensor may be the cause.

So what is an oxygen sensor in a car? Read on as we explore how these clever sensors work to save you money and help the environment.

What is an oxygen sensor?

An oxygen sensor, also known as an O2 sensor or lambda sensor, is a measuring probe that monitors the car’s air-to-fuel ratio and relays the information to the car’s ECU (engine control unit). The ECU can then regulate the air and fuel mixture so that your car runs as cleanly and efficiently as possible.

It’s actually a type of battery known as a ‘galvanic cell’. It produces a small voltage which differs depending on the level of oxygen in the exhaust. There are a few common types, but all of them play the same role in the engine, which is to ensure it runs as efficiently as possible by maintaining an ideal air-to-fuel ratio.

What does an oxygen sensor do?

Your car needs to have the correct ratio of oxygen to fuel in order to run efficiently.

If your car is running with too little oxygen, this is known as a ‘rich’ fuel mixture which results in your car producing extra pollution because of all the unburned fuel, as well as being worse for fuel economy – and reduces engine power output.

If it’s running with too much oxygen, it’s considered ‘lean’, which can spell disaster for your car by damaging its catalytic converter. The higher temperatures can also melt your pistons and spark plugs – which amounts to costly engine repairs!

Although the ratio will fluctuate a little while you’re driving, running too rich or lean is bad news for your car, and can cause big problems for your engine.

Where is the oxygen sensor located?

The oxygen sensor (or sensors as there’s usually more than one) sit on the engine’s exhaust. They can sit between the exhaust manifold and the catalytic converter, or between the catalytic converter and the exhaust pipe. The first are called pre-catalytic converter oxygen sensors, and the second are called post catalytic converter oxygen sensors.

Symptoms of faulty oxygen sensor

Like any car part, oxygen sensors have a limited life span. Things like soot buildup, oil, salt, dirt, or contaminated fuel can all lead to them failing early.

There’s a few common symptoms that can point to a faulty oxygen sensor, so let’s take a look at them:

If you leave it too long, a faulty oxygen sensor can lead to problems with fuel economy and efficiency, as well as causing engine damage, so it’s a good idea to know how to test it.

How to Test an Oxygen Sensor

There’s a few different methods for testing them that you can do yourself if you like a bit of tinkering. Before each test, take a look and make sure the wires aren’t damaged and there’s no corrosion on the connectors. If you’re ever unsure, bring your car into your local mycar workshop and we’ll give you a hand.

How to test oxygen sensor with multimeter

If you own a multimeter, you can easily test current, voltage, and resistance. You can perform the voltmeter test below using a multimeter, but as the voltmeter only tests voltage, you’ll need a multimeter for this test which checks the heating element. If your oxygen sensors don’t have a heating element you can skip this step.

1. Locate the oxygen sensor you’d like to test, noting down which wires are which. Your owner’s manual should tell you where these are found for your specific vehicle, as well as having a wiring diagram. The sensors will have between 1-4 wires depending on their type.

2. If the oxygen sensor is near the catalytic converter, you’ll need to jack the car up, making sure it’s safely secured and blocked behind the back wheels.

3. Disconnect the wires from the harness, and pull apart the housing to expose the metal pins inside that the wires connect to. Be careful as the exhaust and areas around it will be hot!

4. To test the heater element on the sensor you’ll be measuring resistance, so set the multimeter to 200 ohms and connect the multimeter probes to the pins that the heater wires were connected to.

5. If you don’t get a reading the heater element is broken, but if you get a reading of about 5-6 ohms it’s working properly. All cars are a bit different though, so check the specs in your owner’s manual.

How to test an oxygen sensor with a voltmeter

If you’re using a voltmeter, you’ll be testing the volts with a 10-megaohm impedance digital voltmeter.

1. Warm your car up to operating temperature either by driving around for about 20 minutes, or idling the engine at a fast idle for the same amount of time.

2. Turn off the engine, leaving the ignition switched on.

3. Locate the appropriate oxygen sensor as you did with the multimeter test, and unclip the wires from the harness.

4. If your sensor has 1 or 3 wires, connect the red probe on the voltmeter to the signal wire, and ground the black probe on your car’s chassis. If it has 2 or 4 wires, connect the black probe to the ground wire pins on the sensor.

5. A good reading will oscillate between around 0.1 to 0.9 volts, so if the reading is going up and down like this then it’s a good sign.

We make replacing a faulty oxygen sensor easy

If you’re after professional servicing for your oxygen sensors, your mycar mechanic provides quality service, testing, and replacement of parts Australia-wide. Plus, if you’re looking for peace of mind, our workmanship guarantee means that you’ll always drive away with confidence!

So what are you waiting for? Book a service today at mycar and our team will have you back on the road in no time!