# Automotive Technology/O2 Sensor
## Overview

An oxygen sensor (O2 sensor) measures the amount of oxygen in the exhaust gases, sending the signal to the engine computer. A front oxygen sensor is installed in the exhaust manifold or in the front exhaust pipe before the catalytic converter. As you know, a catalytic converter is a major part of the vehicle's emission control system.
A rear oxygen sensor is mounted in the exhaust pipe after the catalytic converter. See the photo showing how the rear oxygen sensor looks inside the exhaust.
Cars with a 4-cylinder engine have at least two oxygen sensors; V6 and V8 vehicles have at least four O2 sensors. The engine computer (Powertrain Control Module, or PCM) uses the signal from the front oxygen sensor to adjust the air/fuel ratio by adding or subtracting fuel. The rear oxygen sensor signal is used to monitor the performance of the catalytic converter.

## How Oxygen Sensor works

There are a few types of oxygen sensors, but to keep it simple, we will only consider voltage-generating oxygen sensors. As the name implies, a voltage-generating oxygen sensor generates a small voltage proportional to the difference in the amount of oxygen inside and outside of the exhaust.
When the air-fuel mixture entering the engine is lean (less fuel and more air), there is more oxygen in the exhaust and the oxygen sensor will generate a very small voltage (0.1 - 0.2V).
If the air-fuel mixture becomes rich (more fuel and less air), there is less oxygen in the exhaust, so the oxygen sensor will generate more voltage (around 0.9V).
To work properly, an oxygen sensor must be heated to a certain temperature. A typical modern sensor has an internal electric heating element that is powered by the PCM.

## How often to Replace Oxygen Sensor

Oxygen sensors are very important engine management components and are a critical part of the vehicle emission control system. That’s why checking and if required replacing a worn out or ageing oxygen sensor should be an important part of every routine service. A faulty, worn, underperforming or damaged O2 Sensor can:
* Lessen engine performance, causing vehicle coughing, surging, hesitating, rough idling or stalling. Lack of power and poor driveability may also result.
* Increased fuel consumption. Failed or poorly operating oxygen sensors can cause a substantial drop in fuel economy
* Increase dramatically exhaust emissions
* Cause Possible engine damage
* Cause Catalytic converter failure
* Activate the Check engine light
Oxygen sensors should be inspected and replaced at the manufacturers recommended service intervals. Vehicle manufacturers recommend periodic inspection and replacement of oxygen sensors. Typical maximum service life:
* 1 or 2 wire Oxygen sensors - have a typical service life of 50,000 km
* 3 or 4 wire Oxygen Sensors (Thimble type) - have a typical service life of 80,000 km
* Planar Type (Thick film) Oxygen sensors - have a typical service life of 150,000 km
150,000 km sensors are often prone to a shorter life span, and decreased performance, than expected. Where possible all sensors should be replaced at the same time. It is highly recommended changing sensors as pairs. (That is BOTH PRE-Cat or BOTH Post-Cat sensors at the same time)