Torque Shop

Recently, an orange sign lit up on my car's instrument panel.

When I checked the owner's manual, it turned out to be the "Check Engine" warning. I took my car to an authorised service centre.

A workshop technician plugged in a device under the dashboard and found that a faulty oxygen sensor had triggered the warning. What does this sensor do and what happens when it is faulty?

The oxygen sensor, also referred to as the O2 or Lambda sensor, is an important element in an engine's electronic control system.

In most modern cars, there are two sensors - one in the exhaust manifold and the other located in the exhaust pipe after the catalytic converter.

The oxygen sensor is designed to continuously send a voltage to the electronic control unit (ECU) that varies, depending on the amount of oxygen in the exhaust gas.

Programmed in the ECU is the reference value of the ideal voltage, which is used to compare with the value coming in from the O2 sensor.

If the ECU evaluates any difference between the desired voltage and the sensor voltage, it will immediately initiate precise corrections to ignition timing, amount of fuel injected and valve-timing (if variable type) so that the ideal conditions of combustion mixture continue to be maintained. Hence, the ECU is able to constantly optimise fuel consumption.

A defective manifold O2 sensor will send the wrong signals to the ECU, which will no longer be able to determine the ideal set of parameters for the best fuel economy. Often, your car's fuel consumption gradually increases as the sensor begins to deteriorate and this could happen long before the warning light comes on.

The second O2 sensor - the one downstream of the catalytic converter - monitors overall emissions and the condition of the converter.

Although a failed O2 sensor does not cause any major damage, it may affect the overall performance of a car.


A version of this article appeared in the print edition of The Straits Times on January 14, 2017, with the headline 'Torque Shop'. Subscribe