what?

O2 is oxygen. I take it there is a problem with an O2 sensor?
This sensor (also known as a Lambda sensor) is fitted in the exhaust pipe to monitor the amount of oxygen in the exhaust gas, the information from it is sent to the onboard computer, which uses the info to calculate and control the amount of petrol to be injected.

Older vehicles only had one sensor, fitted before the catalytic converter. Newer models have two sensors, the second is placed after the cat and is used to monitor the cat performance.
Engines with a 'V' configuration will usually have seperate sensors for each bank of cylinders.

fred
March 2012