Hi I have a 2001 Outback just turning 120K miles. It has been a great car with ZERO problems. This is the first. My CEL came on while driving and has not gone out. The gas cap is fine, so my guess was to think the O2 sensor ( s) My understaning they need to be replaced eventually anyway. How many do I have? I belive I see the sensor on the cat under the car, but is their another one on the manifold? Second part to the question is that I have a minor exaust leak at the gasket which conects the Cat to the pipe. Could this being throwing the emissions control off? I tried using the HIGH heat metal copper silicone that pep boys sells to seal up the leak. It worked for about 2 weeks and then started to come off? Any ides?

There is a second cat just back of the first one. The second (downstream) O2 sensor is there.

Yes a leak just where you describe can disturb the O2 sensors function. But after the mileage you quote, a new one would'nt be a luxury.

Other than going to a mufflet shop to maybe have a new flange welded to the cat, I dont see how you could devise a permanent cure to your leak.

But others might have better ideas.

Good luck!

You can't just go replacing every sensor in the car every time a MIL comes on. Take it to AutoZone or Advance where they'll retrieve the stored troublecode for free. </rant>

 

I would like to second what Frag said. An exhaust leak before the rear O2 sensor is likely to cause the "catalytic converter performance below spec" code. Flange repair / replacement is your best bet. Also you can replace the gasket with a new one.