Can someone explain to a moron like me what this thing is and what it means when it goes out? I see a lot of talk about it, but not what it is! Thanks!

 

It's a metal cylinder with a couple of holes in it, maybe an inch and a half or two inches in diameter. It threads into a hole in your exhaust system.

It consists of a catalyst (metal) that has a special quality: it produces voltage. Most new ones have heaters to get the O2 sensor up to speed in a hurry.

Basically, air coming out of your engine has chemicals in it, unburned fuel etc. The air flows through the o2 sensor, which "reads" the chemical composition of your exhaust gases. The catalyst reacts with the gases to produce a voltage, which your car's computer then reads.

The computer uses this info. to determine the best air/fuel ratio for your engine, and adjust timing, fuel flow, pulse width, etc. to maintain stoich AFR.

Most new cars have 4 O2 sensors!

If yours go out, the computer will not be able to figure out the proper air/fuel ratio. Usually this results in a check engine light and also (if you use a theoretical system with 1 o2 sensor) you will smell a lot of unburned gas (strong gas smell from exhaust.) If this O2 sensor is not replaced, the catalytic converter (part of your exhaust system) will "chunk" or go bad from all the unburnt fuel.