I have a 3.3L V6 MPI. My engine light came on the hwy.. I brought it to my mechanic and he stated that it was an oxogen sensor. he stated that the codes was flickering so he reset the computer and put some additive to in the gas..One day later the light came back on..back to the garage I went..he went and changed the oxegen sensor..not sure which one but the price was $120...well one day later guess what is back on..going to bring the car back in soon..the code is 134..I did notice that the light does not come on on low speeds but as soon as I reach about 80km it comes on then stays on..the mechanic told me that it looks like the oxogen sensors were replaced before..only have about 120 000 km on the car...runs great other wise..I also have noticed that there may be an exhaust leak somewhere(not loud but I can hear the diffence)..any thoughts before I start replacing varies parts?
I asked this question on another site and got this answer:

Have the catalec converter replaced this is why it does it at high speeds and when your "Mechanic" resets the code it runs fine for a while. This is also the reason that you have a small exhost leak. The converter is begining to plug up. Do this soon as this is very dangerous. It has caused cars to catch on fire. I would also probably change mechanics. This is something that any mechanic should have picked up on immeadiately.
I did bring my car back to the mechanic and he stated that the sensor was put on the wrong spot and put it in the right spot now...I have not seen the light come back on since..but now I am wondering if I should get the catalec converter looked at (possible fire hazard?)..Is there a way to see if it is indeed beginning to plug up?