2000 Honda Civic O2 sensor

It is the primary measurement device for the fuel control computer in your car to know if the engine is too rich or too lean. The O2 sensor is active anytime it is hot enough, but the computer only uses this information in the closed loop mode. Closed loop is the operating mode where all engine control sensors including
the Oxygen sensor are used to get best fuel economy, lowest emissions, and good power and yes the check engine light usually comes on.

-Rob

Hello,

The oxygen sensors job is to measure the oxygen required to burn any fuel remaining in the exhaust stream and relay that information back to the computer PCM (Powertrain Control Module) where it is compared with other live information so that adjustments can be made to maximize fuel efficiency and power via proper air-fuel mixture and ignition timing in the engine.
The oxygen sensor does this by chemical reaction inside the sensor itself.
Oxygen sensor can impact the fuel efficiency of a vehicle negatively if it is not functioning properly.

Here is a helpful guide on Oxygen sensors:

https://www.2carpros.com/articles/how-an-oxygen-sensor-works

Depending on the scenario it can affect the engine idle if the mixture is too rich or too lean.

Faulty oxygen sensor will trigger check engine light. And by scanning the vehicle for trouble codes it can help determine the issue.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

I have attached diagrams for oxygen sensor location and replacement procedure below.

Please let me know if you have any questions.
Thank you.