Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
Like Cheryl said, if you can get your hands on an laptop and a J-2534 device, you stand a chance of doing it yourself. ACDelco TDS is free for anyone to sign up. A programming subscription is $50 after taxes and such.