Oxygen (O2) Sensors

What They Do, and How They Fail

Everyone has had a check engine light pop up, and thinks to themselves “man I hope that’s just an oxygen sensor.”

Everyone has a cousin, uncle, sibling who kinda-sorta knows about cars and says “ah don’t worry about it, probably just an oxygen sensor.”

Little known fact about oxygen sensors - oftentimes they will set a code because they are doing their job and detecting an issue with how your engine is running. It isn’t always them failing.

Modern vehicles have an oxygen sensor before and after each catalytic converter.

4 cylinders will have 2, inline engines will usually have 2, V6-8 engines will usually have 4.

These are called upstream (before the cat) and downstream (after the cat).

What O2 Sensors DO

Oxygen sensors measure the air/fuel ratio leaving your exhaust manifold, and again leaving the catalytic converter. Gasoline engines run best at a 14.7 air to fuel ratio. Too much air is considered lean. Too much fuel is called running rich.

The upstream sensor is going to relay the air/fuel ratio coming out of the exhaust manifold to your ECU/ECM. (click to learn more about the ECU)

If the o2 sensor measures lean it sends a low voltage signal to the ECM, which then makes adjustments to add fuel.

If the o2 reads rich, it sends a high voltage signal to the ECM to decrease fuel.

This sensor usually fluctuates between .1 and .9 volts as it communicates back and forth with the ECM to get the air/fuel ratio right. If it is operating outside of that zone or towards either end for too long, something is wrong.

This is where o2 sensors get their bad reputation. If the engine isn’t running properly and the ECM can’t adjust the air/fuel ratio as it desires, sometimes a code will pop up saying high or low voltage oxygen sensor 1, or bank 1 sensor 1, etc.

The sensor detected a faulty air/fuel mixture and informed the ECU. The ECU couldn’t make the required adjustments so now your Check Engine Light is on.

In this scenario, your oxygen sensors did their job and the code set is pointing us towards where we need to start the diagnosis.

Downstream O2 Sensor and Catalytic Converters

So after passing the upstream o2 sensor, the exhaust gases pass through the catalytic converter.

The job of the catalytic converter is to separate nitrogen oxide into oxygen and nitrogen and change carbon monoxide into carbon dioxide by adding oxygen.

We won’t dive far into converters for this article, but after the converter has (hopefully) done its job, the downstream oxygen sensor takes it’s reading.

Downstream o2 sensors generally hover around .45-.5 volts when everything is running properly. This sensor’s main function is to verify the cat converter is working properly.

Into the Technicals

A sensing element (Nernst cell) in the tip of the o2 sensor is usually zirconia, covered inside and out by platinum electrodes, and wrapped in a porous metal shell. Air from outside the engine flows into the core of the sensing element, while the outside is exposed to exhaust gas via tiny holes in the element’s shell.

The element is measuring the difference in oxygen content between the air and exhaust. This only works properly if the sensor is 600 degrees Fahrenheit or more, so most o2 sensors also contain a heating element, to warm the sensor up before the exhaust gas has reached that temperature.

The difference in oxygen content between the inside and outside of the Nernst cell causes oxygen ions to flow through the cell, which produces voltage.

That voltage then travels through the wires on it’s journey to inform the ECU what is going on in your engine.

How to Test an Oxygen Sensor

You'll need a multimeter capable of reading ohms (or resistance).

DO NOT ATTEMPT TO TEST A HOT O2 SENSOR, WAIT FOR THE VEHICLE TO COOL OFF. (I told you earlier, 600 degrees F. Doesn’t sound comfortable for your fleshy parts, eh?

Most modern heated oxygen sensors will have 4 wires. The 2 same color wires should be for the heating element inside the sensor.

• Disconnect the sensor and touch your leads to the pins that correspond with the same color wires.

• Set the multimeter to 200 ohms setting.

• If it comes up as infinite/open resistance, the element is broken and it’s time for a new o2 sensor.

Heating elements breaking is the most common failure of an o2 sensor, and will cause them to set codes and illuminate the check engine light.

To test the probe element, life is exponentially easier with a scan tool that can read live data.

There are many opportunities to get burned or hit a moving part while your engine is running, so I don’t recommend checking anything more than the heating element at home without a scanner.

If you or a buddy has a scanner that can read live data:

• Hook up the scanner

• Read your engine’s live data, specifically monitoring the oxygen sensors

• Look for wild fluctuations below .1 volts and above .9 volts on sensor 1

• Look for sensor 2 to hover around .45-.5 volts

• If a number is off you’ll need to add fuel or air to the intake to see if the sensor responds with a lower voltage as you add air (remove the intake duct) and a higher voltage if you add fuel (spray some brake clean or starting fluid into the throttle body)

• If the sensor responds accordingly, your problem is elsewhere and further diag is needed. If it doesn’t you can confidently replace the faulty o2 sensor.

Well Then

Now that you have a better understanding of what an oxygen sensor is and does, hopefully we can all stop blaming it for every check engine light we ever see.

If you determine you have a faulty o2 sensor, for most vehicles all you need is an oxygen sensor socket, a ratchet, and perhaps an extension to change it out.

This is usually an easy enough job to do at home, but as always, comments and DMs are open if you need some assistance, hit me up @BowTiedGreaseMonkey