catalyst unit in the exhaust I mean . some time ago Steve has posted something about taking the blue co2 testing cap off in order for the catalyst to breathe; what does that mean ?Leave it off? How do I know that the catalyst was plugged or not ezxcept the funny smell ? Is this the blue cap on top of the intake manifold , my car was running extremely rich for awhile due to the vacuum leak. Replaced (packed with soot) co2 sensor after tackling the problem, but I'm wondering about catalyst it smells a little bit still - no more black smoke out of the pipe , nice smoth idle. the car is 184 k already ,would it be the time to put the metal rod thru catalyst and convert it thus into another pipe ?

LOL seriously.. to see if its the cat without removing it..
1) what does it smell like? Sulfur? Rotton eggs? = bad cat
2) Check airflow at the tail pipe if its weak then pull the blue cap by the intake.. if there is alot of pressure there then you prob got a plugged cat.