I had a Corolla with a clogged CATCON. First indication is that when you climb a slope such as at Kallar Kahar, you feel engine is giving less power (odometer reading was 40K, I had bought the car new).
First I thought engine has become weak. Kept driving like that for many months. Then when odometer reading was 52K, car was really slow in climbing hill, and could barely touch 120 km/hr on motorway. Fuel consumption also increased.
Showed to many so called experts, all of them misguided me. Somebody said change clutch, somebody else said tapet work is required. Then did research on Internet and decided to check out CATCON.
Went to sarya chowk in Islamabad, and got CATCON cleaned from Imran silencer walla. It was fully choked with orangish stuff. All he did was opened it up and cleaned with high pressure air. After that full engine power and fuel economy were restored.
Later I realised it was fuel of Attock pump, which caused it to choke. Now I never go to Attock, always to Shell.