2003 Volkswagen Passat Check Engine

I have had this car for just under a year now. In that time i have replaced 3 out of the 4 ignition coils. Yesterday the chekc engine light came on and the car began to shake and i thought it was for the last coil, so i changed it. That was not the problem. Then i thought it was the spark pulgs. therefore i change all 4 of them as well. Unfortunatlly the problem is still not fixed. I was wondering if there is any direction you might be able to lead me to in what the problem or solution might be. And how much it would roughly cost. Thank you