There are 4 million reasons why your check engine light comes on. You need to take it up to a shop and have it hooked up to a diagnostics computer which will read out a code and thus the mechanic will determine what is wrong and be able to tell you.
When they replaced your parts, they undoubtedly unplugged some diagnostic sensors. When doing so it automatically ignights the check light. They should have reset your car's computer. Its very easy to do. Check a Haynes manual. It'll walk you thru it. It takes about 5 minutes. Get a manual at most parts stores. Costs around 10 bucks.