If you have the computer scanned, it will tell you which of the oxygen sensors has failed. We doubt it. When an up stream oxygen sensor is faulty, it may cause an over-rich condition that produces more horsepower. This, of course, caused higher emissions. With the new oxygen sensor, this condition was corrected.
Oxygen sensors read the fuel mixture in the exhaust and send the information to the computer.
The computer then, adjusts the fuel mixture. By not replacing faulty oxygen sensors, your can expect that your check engine light will eventually come on accompanied by erratic idle and poor running. Your car probably will not quit running and there should not be any damage done to it by driving it with faulty oxygen sensors. MOT failure on emissions - running at 10.30. Garage suggested several possible problems starting with Oxygen sensor - cost of repair 100.