96 Camry - Engine Check Light on

I had worked on exhaust on this car two years ago. There was some leakage that still is there. I replaced radiator by after market radiator last week. Every thing seemed alright for a day. Then all of a sudden engine check light came on. It comes on when I start whether cold or warm and stays on. Could there be some restriction in radiator or may be transmission oil may be too much that is affecting sensor? The car has 126000 miles on it? This a 4 cylinder automatic with air conditiong. Any help will be greatly appreciated? I am not very experienced but if it simple, I can try.