how would you know if the oxgen sensor is failing?

[initial s14]

would it still fail even if the engine light doesnt give me a sig? how often do i need to change the oxygen sensor on a car (by mileage) and do i need to change it still regardless? I just feel that after the tune up i am still eating more gas. Or is that how an 240sx (s13) is with a 27-21 gas mileage consumption?

[punxva]

start the car, and warm it up, run the engine at about 2,000 rpm for about 2 minutes under no load, turn the little flat head screw on the ecu counter clockwise and hold it there for 2 seconds, and then rev it 2,000 rpm and hold it there, your o2 is working if the red light on the ecu goes on and off more than 5 times during 10 seconds. Then to get out of that mode, just turn the car off.