@tashkeel bhai as far as i know and heard from many experts. when you accelerate tha car it throws carbon from back and other major problem the car wont accelerate as its meant to be. you will feel the power loss in car and some non pro mechanics would tell you your clutch is out or things like this but major power loss is observed. civic new catalytic costs more than 70,000 and its just because of poor fuel. and its very common, if the car is kept good then its recommended to be cleaned every 15000Km in Pakistan.
May be more experts will clear your point. my old civic had this problem and i was like in hell. if its blocked your engine cannot throw the hot airs out so many problems for engine tooo and if some one purchase a car with block catylatic i bet he will say civic is not a good car casue its like very very important part. many break the catylatic but it makes strange noises and its not recomended.