Oxygen sensors 3

[matgray]

Hi, most modern cars have two oxygen sensors on them,one usually in or close to the manifold for adjusting the air fuel mixture accordingly, what i would like to know is what the second oxygen sensor is for,as every body i have asked has a different opinion from,its a back up sensor to i havent a clue, i would like to know as i am only 24 and belive that you cannot diagnose a problem with a car un til you completely understand how all the components of an engine/electrical system work.

many thanks. matt

 

[JPARKER70]

i know that on my truck i have a o2 sensor that is refered to as the down stream o2 sensor. it is located behind the converter, and before the muffler. i was always told that those are use to check the effectness of the converter and it also a pollution check.

 

[matgray]

by measuring the effectiveness does it alter the fuel to air ratio at all

 

[carnage1]

the second o2 sensor is after the cat and is partly there to ensure the car stays emmisions legal by monitering the cat's performence. other than that I am not sure.

 

[JPARKER70]

i am not a 100% sure if it effects the air fuel ratio or not, but i know for sure one thing. my down stream o2 sensor is burnout right now and my fuel mileage sucks. lost five to six mile a gallon.

 

[ubrales]

The upstream O2 sensor is called the "Control Sensor" and the downstream O2 sensor is called the "Monitoring Sensor". Most people refer to these as "Sensors", hence the confusion.

The Control Sensor sets the Air/Fuel mixture and ensures that the mixture is lean. Further leaning (mixture) will cause combustion problems. Even with this lean mixture, there is a small amount of unburnt fuel (hydrocarbons) present in the exhaust. These unburnt hydrocarbons are burnt off in the catalytic converter and the final exhaust is monitored by the downstream "Monitoring Sensor" which also checks for proper function of the cat convertor.

 

[Doug020668]

The requirement is to have two sensors these days so as to comply with emissions regulations etc. The system that uses 2 sensors is known as On Board Diagnostics II (OBDII)

The upstream one is monitoring the exhaust and providing the necessary info for the ECU to adjust the Air/Fuel Ratio, the downstream one monitors both the performane of the catalytic converter, and also the performace of the 1st sensor. If the cat fails, a fault code will be set, and if the 1st sensor fails, the check engine light will come on and the second sensor will sort of substitute for the 1st one so that the engine can be "Limped" home.