How can you usually tell if your o2's are going bad?

[Lunatikgixxer]

Lately when i shift at high RPMS at about 6k or so my car will do one decent backfire everytime i shift. I was thinking i havent replaced my o2s and ive had the car for a year and half and who knows if or when the last owner replaced them. Anyway to physically look at them and tell if theyre bad. I have no codes thrown but i am just concerned. I also noticed a little bit more of a fuel smell lately. Could it be caused by a bad spark plug?

[pajeff02]

They will generally throw a code when something is really messed up, however, their performance does degrade over time. Easiest check is to connect a scanner and watch the live sensor data from the front O2's (the rears are only there to tell the PCM if the cats are working properly and have no effect on fueling). The data, which should be read in millivolts, should rapidly cycle up and down as the car is running. If one sender's output is stagnant, or significanty slower than the other, that means it is time for it to be replaced.

[Lunatikgixxer]

ok i have a scanner ill check this out in a minute, what numbers should i look for?

[pajeff02]

I believe around 400 mV represents a stoichiometric A/F ratio, however, you will never actually see this number. It should cycle across a couple hundred mV very quickly -- the data output from these narrowband sensors is really unreadable to anything other than the PCM.