Back again with a new code.. P0420 Catalyst System efficiency Below threshold bank 1. As I understand this has something to do with Upstream O2 sensor.
However I am suspecting a exhaust leak between exhaust manifold and the cat converter. I have replaced the cat converter 3 weeks ago along with the downstream sensor. When I turn the car on I can hear the leak but I have a tough time figuring out where exactly it's coming from. I can really hear it on cold startup..

Is there an easy way to figure out where the leak is coming from? I did replace the gaskets for the exhaust manifold.
Thanks

FYI this is on a 2006 Volvo s40 2.4i

 

Check the exhaust manifold nuts. Mine made a sewing machine sound when cold that went away once warmed up.

I got rid of my p0420 with a Cel Boss from IPD. Basically it backs the bottom O2 sensor away from the exhaust flow. This may not be legal in your state.

 

 

The upstream sensor won't be reporting on the cat's efficiency, but is used to set the fuel/air mixture. Of course, if it's being misled by a leak in the exhaust system, it might be dumping too much fuel for the cat to deal with.

In the end you really need to find that leak - it could easily be the source of your problems and the OBD code. It might help to use a length of tubing as a stethoscope - just stick one end in your ear (loosely), and wave the other end around the suspect areas. When you get on the leak, it'll be pretty obvious. Then fix the leak, reset the code, and see what happens.

I've had reasonably good luck using a simple spacer on the downstream O2 sensors (moves it out of the exhaust flow, which can improve the reading - from a "pass the dang test" perspective, that is). ;-)

 

Ok that makes sense now... This leak could also be a cause for my Lean condition if the sensor is sending too much gas into the engine. I will try to find the leak later on. I will update this thread soon

 

quite often there can be exhaust leaks around the manifold (loose or broken off studs/nuts) or around the flex pipe - I'm on my 3rd flexpipe for a 2000 S40 (owned since new). Note that the P0420 code uses the difference between the upstream O2 sensor (pre-cat) vs the downstream O2 sensor to measure the "efficiency". So to add to the possible causes, it could be that in fact the cat is going bad (themal imaging may be required to see if its getting up to proper temp) or the downstream O2 sensor may be off. I agree with Habby and Hudini that right way to go is to check for any air leaks (intake and exhaust) before throwing money at sensors and cats. If you need to pass emissions in a hurry, then you can look at inserting a "cel-boss" spacer as noted.

 

I had to get a new cat. The one that was on there the mechanic said that it was universal and did not work the way it's suppose to. There was also a leak on the mesh just above the cat. New cat is on and all looks good for the exhaust part.

 

Sounds like it was a twofer - both a leak (which can cause a problem) and a bad cat. Gotta believe it's going to work fine with a new "good" cat...