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How to Test O2 Sensors

Omkar Phatak
What are O2 sensors and how to test them? If that is the question bugging you, here are some tips explaining how it's done.
An O2 sensor is used in car engines to monitor the oxygen level in exhaust gases and compare it with its levels in the environment. They are also known as lambda sensors.
They are the crucial link, in a feedback mechanism, which monitors the air-to-fuel ratio in the car engine. This ratio is the factor which decides the efficiency and exhaustion levels of the engine. The optimum or ideal ratio of air to fuel in the engine, for proper operation, is 14.7:1.
This ratio of the fuel available for consumption, and the air available for burning in the engine, varies over the running of the car. Nowadays, a mini electronic computer controls all aspects of an engine's functioning. The eyes and ears of this computer are the sensors, like the O2 sensors, which form its feedback mechanism.
Oxygen sensors were first invented and brought into use, in the 1960s, by the Robert Bosch GmbH company, under the able leadership of Dr. Gunter Bauman.
Originally, it was a zirconium ceramic and platinum coated device, shaped in the form of a thimble. The newly developed planar type sensors were also first invented by the same company, and are widely used in automobile engines, all over the world.

How Do O2 Sensors Work?

The sensor is made up of a ceramic cylindrical body, fitted with platinum electrodes. This whole assembly is covered by a metal gauze for protection. It operates on the principle of a Nernst cell.
The O2 sensor probe becomes operational only at high temperatures and therefore, requires a separate heating mechanism. The electrode voltage is dependent on the difference in oxygen levels between the exterior atmosphere and levels in the exhaust gases. This is the principle that is exploited in monitoring the oxygen level.
The output voltage levels are calibrated according to the comparative oxygen levels. There are two extreme voltages that indicate two extreme ratios of air and fuel content.
If the output voltage of the electrodes is 0.2 V, that indicates the air-fuel ratio is just enough so that all carbon monoxide gets oxidized and carbon dioxide is released. This state is low on unburned fuel and rich in oxygen. As opposed to this, the other extreme is reached at 0.8 V, which is an oxygen deprived, unburned, fuel-rich state.
The optimum engine performance state is indicated, when the sensor output voltage is 0.45 V. This is the state where the oxygen content and fuel is in equilibrium and the pollution caused through exhaust gases is lowest.

How to Test Oxygen Sensors?

There is a simple way of testing an O2 sensor. Take a high impedance digital multimeter and attach its negative lead to the output of the sensor. The sensor needs to be connected with the car control computer, during the testing.
Now rev up the engine to around or above 2000 rpm, for the sensor to get heated up and operational. An indicator of a good sensor is that it keeps on crossing above and below 0.45 V rapidly.
If the voltage levels keep on rapidly oscillating through high and low levels, that is between 0.1 and 0.9 V, the sensor is functioning well. Now, use the choke, which introduces excess fuel in the engine, and check if the voltage output goes towards the higher side, around 0.7 V. If that is so, the sensor is doing its job and seems to be working properly.
Thus, you have to physically check the output voltage levels, in both the extreme states of engine operation. If it shows the right voltages at both extremes and oscillates between the two in optimal state, it's working properly.
Today, with the levels of air pollution, poisoning the air we breathe, it is important that you keep a tab on your car engine condition and monitor its emission levels. Regular testing is therefore, an essential step in car maintenance.