How to test Oxygen sensor | FULL GUIDE

Forget BS guides to test the O2 sensor. Before I had to test mine, I watched and read a bunch of confusing content online. Now after doing it myself on 2 cars, I figured out the easiest ways to test your O2 sensor, and here is an exact step-by-step guide to do it.

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VIDEO: How to test Oxygen sensor

What is an Oxygen sensor?

The oxygen sensor measures the amount of Oxygen in your exhaust and sends this data to your ECU. There are 2 types of O2 sensors:

Upstream vs. downstream oxygen sensor

Upstream (pre-CAT): The oxygen sensor before CAT monitors the amount of air, and based on that ECU will adjust amount of fuel to achieve a perfect air/fuel ratio.

Downstream (post-CAT): This sensor is also measuring the amount of oxygen, but this data is used to see if the catalytic converter is working properly.

Oxygen sensor types

1-wire sensor

2-wire sensor

3-wire sensor

4-wire sensor (narrowband O2 sensor, most common)

5-wire (wideband sensor, also called Air/Fuel ratio sensor)

Locating Oxygen sensor

To locate the Oxygen sensor simply start from the engine block and follow the exhaust. The first Oxygen sensor is sensor 1. The second oxygen sensor is sensor 2, or if you have more of them the sensor number means its position from the engine block to the end of exhaust.

Oxygen sensor wiring diagram (4-wire)

This is not 100% diagram used on all vehicles but most commonly the wiring will have these colors:

2 wires with same color (black, white, etc…)	Power and ground for heater element
Black wire	Signal output wire
Gray or other remaining wire	Ground for signal output

How to test the O2 sensor

You can test the Oxygen sensor with either a multimeter or an OBD2 scanner. Even watching live data is enough to check if your O2 sensor is working and even if the ECU is working and evaluating O2 sensor data correctly.

However for some tests, likethe heater for the oxygen sensor, you will need to use a multimeter as well.

How to test the Oxygen sensor with an OBD2 scanner

Almost any OBD2 scanner offers the option to view oxygen sensor data and even display them in the form of graphs.

It is also easier if your scanner supports data logging, so you don’t have to look at scanner while driving.

Test upstream oxygen sensor with live data (narrowband sensor)

For the most common 4-wire sensor (upstream), you want to see it oscillate between 0.1V and 0.9V when iddling.

For testing if it can pickup changes in amount of Oxygen, you can simply press gas pedal for a second and let go, there should be some response in live data.

Test upstream oxygen sensor with live data (5wire wideband sensor)

The wideband sensor signal will not jump up and down like the 4-wire sensor. It will be straight and to test if it can catch changes in air/fuel ratio, simply step on the gas pedal for a second and let go. There should be a change in your data on the OBD2 scanner like in the photo below.

Test downstream oxygen sensor with live data

You can do this simple test while driving and logging O2 sensor data to test your downstream sensor.

• Drive at Around 1500 RPM
• Go full throttle for 3-4 seconds
• Put your foot off the gas pedal and let the car break by itself

If a sensor is good, you will see it running high when accelerating and dropping to 0V when you take your foot off the gas pedal.

How to test an Oxygen sensor with a multimeter

With a multimeter, we can do the following tests:

Test	Leads placement	Multimeter setting	Test result
Power for heater	Black on battery- Red on heater power wire	20 V	12V with key ON in some cases less e.g. 4.4V But not under 1V.
High resistance in ground	Black on battery- 1. Red on ground wire for the heater 2. Red on ground wire for signal	200 Ohm	Under 1Ohm is good ground.
Heater resistance	1 lead on heater power 1 lead on heater ground (sensor unplugged)	200 Ohm	Try searching for your car but cold heater should have 2-20 Ohm if good.
Signal wire check	Red lead backprobed to signal wire with connector plugged and engine ON. Black lead on good ground, for example Battery-	20 V	Oscillating between 0.1V and 0.9V when engine iddling.

Check for power

To measure power for the O2 sensor heater, place the black lead on the negative battery terminal or other ground on the vehicle.

With the key ON, try all 4 pins on the O2 sensor connector (you probably dont know which one is for which wire) and the power wire of the heater element usually shows battery voltage (e.g. around 12V).

In some cases like this Fabia on picture, the 4.36V is also OK because most sensors are operated on this voltage reduced by ECU.

If you don’t see voltage above 1V in any of the wires with the key ON, this is a problem with the heater circuit.

Measure resistance for heater element

Typically, when cold, an oxygen sensor heater circuit might exhibit a resistance of a few ohms (e.g., around 2 to 20 ohms).

It is important to understand that when the sensor heats up, the resistance value will change. It is always best to refer to the specific vehicle’s service manual for the precise resistance specifications.

Check grounds for high resistance

You also want to check the resistance between the grounds on the Oxygen sensor (signal ground and heater ground) and your negative battery terminal.

The expected value is close to 0 ohms, in this case, the 1.5 ohms seems like there might be some ground problem.

You want to see it below 1 ohm.

Check signal output

If you cannot check the signal with an OBD2 scanner, you can test the signal wire with a multimeter. Use T-Pin to backprobe signal wire connector and while the connector is plugged in start the engine.

Use a 2V setting on a multimeter and place red lead on your T-Pin and black lead on any good ground.

Now you should see voltage oscillating between 0.1V and 0.9V if the sensor works.

If you see low voltage (e.g. 0.2V) or high voltage (e.g. 0.8V) and it is stuck and doesnt oscillate try to remove some vacuum line, spray the brake cleaner in the air intake or just step on the gas pedal.

If there still isn’t a change in voltage, your O2 sensor is bad.

How to check the O2 sensor signal wire with a multimeter

How to check the O2 sensor signal wire with a signal generator

You can also check your O2 sensor signal circuit + ECU by using a signal generator. Here instead of Oxygen sensor signal, we are sending fake signal and if signal circuit + ECU work properly, we will see it on OBD2 scanner.

Once I Turn ON signal generation, there should be a response in the Oxygen sensor live data if the signal circuit and ECU work properly.

Tools needed to check the Oxygen sensor