Billing code for colonoscopy - history of colon polyp

What are the options for medical diagnosis codes for a colonoscopy due to history of colon polyp for someone under the age of 50? I'm advocating for myself here. My insurance company keeps denying payment for my colonoscopy because I'm under 50 and my doctor's office keeps recoding but apparently can't find the right code that is for a medical diagnosis and not a preventative code. Therefore, my insurance company won't accept it. Can anyone be of any help here? I'm preparing for a second appeal process. The procedure was done five months ago. Thank you.