legacycentral bbs

I’ve noticed that there seems to be some confusion about O2 sensors on the forum and in general. So here’s how most O2 sensors work. I say most O2 sensors work like this because I’m sure there’s some weird variation out there I’m not familiar with. Ever wonder why it’s called an OXYGEN sensor and not an exhaust or fuel sensor? That’s because the sensor compares the amount of oxygen in your exhaust to the amount of oxygen in the ambient air outside the sensor in the engine bay/underside of your car. Sounds simple right, well here’s were things get weird. They work on a principle called the Nernst Effect which is a theory basically stating that zirconium dioxide becomes electrically charged at 600 degrees F. OK still with me? Well your basic 2 wire O2 sensor is coated with zirconium dioxide, once it reaches 600F it becomes electrified. Now here’s where the sensor begins to compare O2 levels. If there’s lots of O2 (engine is running lean) there is little difference between exhaust gasses and outside air, meaning there is little electron difference between the zirconium dioxide and the outside air meaning there is little electron flow, which equates to a low O2 sensor reading (0.1-0.4 Volts) Now the PCM reads this opens up the injector pulse width and now there is less O2 in the exhaust (engine running rich) less O2 means MORE difference, meaning MORE electron flow, meaning higher O2 sensor reading (0.5-0.9Volts) this signal is sent to the PCM which shortens the pulse width of the injectors and the whole cycle starts over. This is also why a properly functioning O2 sensor has a reading that steadily rises and falls between 0.1-0.9 volts. A hole in your exhaust pre O2 sensor will flood the exhaust with oxygen causing a false lean reading all the time which will cause your PCM to constantly try and richen your fuel mixture, so make sure all your flanges are tight! Now the 3 and 4 wire sensors work the same way only the extra wires are for artificially heating your sensor so it “lights off” (600F) faster. 3 wire grounds the heating element through the sensor, 4wire grounds the element with the 4th wire. Newer sensors use titania which creates resistance, once at operating temp, in response to O2 levels which creates the signals to the PCM which provides a 1.0volt reference to the sensor. The more O2, the more resistance, the lower the voltage that gets through and visa versa. Wideband O2’s work this way but are capable of noticing an O2 level change in 100 milliseconds were as a normal O2 can only notice in about 0.1sec. I know I’m a nerd but I hope this helps someone out!

PS this is how a Pre Catalytic Converter O2 sensor works, the “downstream” post cat O2s are there just to monitor cat converter efficiency, they should only see one O2 level and there for put out a steady voltage, if they are wavering like the “upstream” O2’s that means your cat isn’t doing its job and is letting hydrocarbons out, distorting the signal.

Word to nerdness. That was a great explanation of how O2 sensors work... I did not know that. Thanks for teachin' me something new!

Good stuff to know.

And @ "AKA I'm a nerd." I should put that in all my posts.