I have a 3000GT and in Feb it passed inspection. The 1 week later the check engine light comes on. I bring it in and they say its an O2 sensor. They replace it. Later that day the light comes on again. Bring it back and they replace another o2 sensor. One day later the light comes on again! What the heck is going on here? How many o2 sensors are there? I have already spent $500 replacing the 2 and am afraid I am getting ripped off here. Doesn't the computer give ALL the codes when you first check it? Is it possible all these o2 sensors are going at the same time? The computer code said they are running hot.

Any help here would be greatly appreciated.

What year is it?

The computer will show all faults (Codes)

The mechanic said this is true, but sometimes one fault will "mask" another.
Does this sound true? Anyone else ever have something like this happen?

What year is it?? I've seen in a lot of other vehicles where the car will read the o2 sensor is bad...but really it's the catalytic converter and the o2 sensor is reading higher levels..or running hot like in your case.. just a thought...don't quote me on it

The car is a 1999.

I have a feeling you are correct. He did point out some red or orange spots on the bottom of the catalytic converter that indicated it was running hot. Does this mean I only needed to replace the cat. converter and NOT the (2) O2 sensors? In other words, did I just waste $500 on nothing or would the o2 sensors need to be replaced anyway?

Any idea how much it will now cost me to have the Cat. converter replaced?

go to autozone or something and take a look at cats there....they may not even carry exhaust components for these cars...but it's not really BAD that you changed the o2 sensors...just a step ahead..thats all