If the exhaust system was replaced from the cat. back it should not have anything to do with the check engine like coming on. Unless there was a O2 sensor in the exhaust pipe that was installed new. If it was left off or not plugged back in then yes this could be your problem. I would take the car to your local auto parts store have them scan the codes for you, It's free. Ask them to reset the codes and make a note of the code numbers, drive the car and see if the light returns before fixing anything.