How can you tell if a O2 sensor is bad
 

Another thread deals with an O2 sensor. Do o2 sensors fail hard or is it a gradual thing. I know a o2 sensor is listed as a tune up item for cars. How many miles or years do they last?

Is this one of the things that causes gas mileage to fall off after a few years?

Is there a way to test one?

Thanks

Re: How can you tell if a O2 sensor is bad
 

Yes O2 sensors do fail but when they do they won't hurt anything. If you don't see that a code is set one of the ways to tell they have failed is that it will run rich at idle or if their readings are very still on the scanner. They should read over a wide range and your crosscount numbers should be frequent. You should always change your O2 sensor with every plug change if you: race, run high octane fuel, after one run with leaded fuel, and the MOST important here. DON'T BUY PARTS HOUSE BRAND SENSORS. Always use the GM, Merc or who ever manufactures your engine. If your engine uses the heated sensor (3-wire) then they can go longer between changes.

Roby

Re: How can you tell if a O2 sensor is bad
 

Ideally, a digital storage osilliscope is used to manitor sensor operation but a plane old digital multi meter can be used. Set it for the 2 volt scale, watch the voltage. A good sensor will constantly vary from approx .2 votls to .9 volts. You should see a change a couple times per second. A lazy sensor may show a higher low voltage and a lower high voltage and switch back and forth at a slower rate. A really bad sensor will show a fixed or nearly fixed low voltage unless it is a heated design and may short internally to the heater power circuit and will show a fixed high voltage. Of course all this is untrue if the engine is running extremely lean or rich as either will fix a voltage at full rich(high) or full lean(low) readings.