my ck enging lite is on .i have put a new catv on and a new flex pipe.also a new on 2002 Buick Century

water pump ,i dont think is the intake manifold gasket cuz egine runs perfact.

Have it scanned at a shop not a parts store .The code(s) will tell you what system(s) are affected and help lead you to the proper fix. If the exhaust repair (catv?/flex pipe) is near an oxygen sensor , check for leaks , even a small one can affect it's readings causing the MIL to illuminate.(MIL= malfunction indicator lamp)Good luck.
thank you very much
1 more answer , 1 more comment
What code are you getting? Important.
cant remember ,but there was 4 choices, 1 was the intake mani. the other was catv. or felx pipe.i paid 329.00 gor both.