2003 STS Po420 code and other issues

[drcmrn]

I have a 2003 STS that I replaced the Cat and still have the P0420 code. i notice a few things going on, one is that my car sometimes runs a little rough when starting and idles at 1200 RPM's for 40 seconds even if the car is still warm from running earlier. i also dit the WOT ten times to reduce oil consumption but it still needs a quart at about every 5-700 miles. the car when warm runs and sounds like a champ. are there other things you can do besides replacing the CAT? what other things can cause a P0420? I appreciate all advice

[Texas Jim]

I have a 2003 STS that I replaced the Cat and still have the P0420 code. i notice a few things going on, one is that my car sometimes runs a little rough when starting and idles at 1200 RPM's for 40 seconds even if the car is still warm from running earlier. i also dit the WOT ten times to reduce oil consumption but it still needs a quart at about every 5-700 miles. the car when warm runs and sounds like a champ. are there other things you can do besides replacing the CAT? what other things can cause a P0420? I appreciate all advice

Did I read this right?

It is using a quart of oil every 500 to 700 miles?

THAT is a lot of oil usage.

I "THINK" that the P0420 code is when the o2 sensor in front of the cat and the one at the rear of the cat detect more emissions LEAVING the cat than is normal.

I don't know the specs on your converter...but...

I am not sure if a cat can handle that much oil usage without throwing a code.

Here is a link about how a CAT works and a little info about the P0420 code.

http://www.aa1car.com/library/p0420_dtc.htm

Hopefully someone can chime in about the roughness and high idle.

I really don't know anything about that.