My car suddenly started acting up yesterday, and it has the same symptoms once before when they failed, but not nearly as bad. This time the car stumbles upon take off, slightly noticable lack of power, and a extremely rich smell. There is no SES though. The previous time they failed it threw a code. I have an air/fuel ration gauge and it is not reading properly.....Basically I need to know if an O2 sensor can fail or start to fail without a code.

The o2's are AC's and are about 2 years old maybe 15-20k on them.

Thanks,
Nick

 

mine didnt throw a code. Same conditions

 

Mine didn't throw any codes. Took me to scan it to find out they were dying. I think it really depends on how bad it is.

 

They only throw a code when they are dead... but if you have a scanner you can see it dying long before its gone.

 

Just what I needed to know, thanks fellas.

 

Interesting, I figured they would throw a code if they were going bad, but it seems only if they actually die and stick at a voltage will they throw a code.

My o2s are original with 97k miles, give good readings it seems from what people said about my scan logs.......anyway, I thought that they were supposed to not read anything when the car starts up cold? Like 450 mV or something until they get hot. Well this morning they were both reading 50-150 up and down with the coolant temp at like 50 F right after starting the car up, and this is in open loop of course. Acted like normal once the car went into closed loop. Is that normal? Guess it is almost o2 time for me maybe.

 

I belive the PCM doesnt "use" the numbers reported by the O2's until warmed up in closed loop.
As for them going bad, the computer will throw a code if the O2's max out or bottom out in their readings. But the code will be LO2/RO2 Lean/rich. Just saying that its reporting that one side is beyond correction in the tables. They can also throw a code if theyre not functioning right (i think).
Finally, if their numbers are "off" they can also be bad, but if they don't hit any of the above situations it won't throw a code until they get "worse".