O2 Sensor Function, Diagnosis, and Replacement

Article by Mark Trotta

Oxygen sensors, commonly called O2 sensors, have been installed in cars and light-duty trucks since the early 1980's. Their function is to monitor the air/fuel ratio of the engine exhaust, then feed the information to the computer.

automotive 02 sensor function

Many technicians do not realize they are considered a 'wear' item. They do not last forever!

Located in the exhaust system, oxygen sensors do not actually measure oxygen, but rather the difference between the amount of oxygen in the exhaust gas and the amount of oxygen in the air. Rich mixture causes an oxygen demand. A lean mixture causes low voltage, indicating there is an oxygen excess.

How Many Oxygen Sensors Does My Car Have?

It varies, depending on year, make, model, and engine. Your vehicle can have one, two, three (or more) 02 sensors.



How An Oxygen Sensor Works

Through the sensor, information on oxygen concentration is sent in real time to the ECM (engine computer) as voltage measurements. There, the readings are coupled with information from other sources, which the ECM analyzes, then adjusts the amount of fuel injected into the engine to compensate for either the excess air or excess fuel reading.

When the ideal air/fuel ratio of 14.7 is obtained, a catalytic converter works at optimum efficiency. O2 sensors that have been modified or tampered with may malfunction, which will can cause an engine to run lean and decrease fuel efficiency. Faulty 02 sensors damage your fuel economy as well as the service life of your catalytic converter.

How Can I Tell an Oxygen Sensor is Bad?

Physical signs of a bad sensor are ceramic chipping and carbon deposits. But what you can't see is wire burnout or an internal circuit disconnection.



An oxygen sensor may not fail completely. While driving, you may notice a slight loss of power, a rougher idle, or a drop in fuel mileage.

How To Test An Oxygen Sensor

A digital multimeter is the easiest way to test for a bad 02 sensor. Connect the test lead to the single wire which the sensor connects to. Start the engine, and wait until the ECM goes into "closed-loop" mode (In field service mode, the "Check Engine" light will change from flashing 2 times a second to once a second).

Your voltmeter readings should be sweeping back and forth over 450 milli-volts, down to 200-300 and up to 700-900. The high reading reflects the a rich condition, the low reading reflects a lean condition.

best digital multimeter for automotive

Shop: Digital Multimeter

If your voltage readings are not going up and down, the sensor may be contaminated. If the readings are high, the sensor wire may have a short. If the readings are low, the sensor wire may be grounding out.

A reading that stays near the middle (450 milli-volts) indicates a possible open short in the sensor wire.


How To Remove An Oxygen Sensor

Removing an oxygen sensor requires a special slotted socket.

oxygen sensor removal tool

Although these are a single-purpose tool, they do fit most years, makes, and models, and are relatively inexpensive.

oxygen sensor removal tool

Shop: Oxygen Sensor Socket

Along with the socket and 3/8" ratchet, you'll probably need an extension.

NOTE: If you were in a jam and needed a tool right away, you can make one from a common six-point, 7/8" (22mm) socket. A notch needs to be cut out lengthwise from the socket to clear the wire.


How To Remove a Stuck 02 Sensor

There's plenty of nightmare stories of oxygen sensors not coming out. The worst case scenario would be having to remove the exhaust pipe from the car, then trying to unscrew the sensor with a hammer and vice grips. Hopefully, this will not be the case with yours!

It's often easier to remove the sensor when the exhaust is slightly warm. This can be done by idling a cold engine for just a few minutes. Or, if you prefer, heat the oxygen sensor bung with a small propane torch (use caution).

Installation is the reverse of removal.

NOTE: Some new oxygen sensors come with the threads already coated with anti-seize compound. Adding a dot of anti-seize lube to the threads will insure it will come off easier the next time.



Since they are considered wear items, oxygen sensors should be replaced within manufacturers recommendations. Intervals range from every 30 to 50k miles to a maximum of every 100,000 miles.

Replacing worn O2 sensors with new ones will increase your car's fuel efficiency.


Related Articles:

Emissions System Components

Catalytic Converter

EGR Valve Function, Diagnosis, and Repair

AIR Pump Function