This notice is generally received by taxpayers that manually prepare and mail their tax return to the IRS. You have received this notice because the IRS has found a miscalculation on your return which in return, has now caused you to have a balance due instead of a refund.
You will first need to compare the notice to the calculations and figures on your return. If you agree with the corrections, simply correct your return for your records and pay the amount due.
If you do not agree with the notice, call the toll-free number listed on the notice within 10 days of the date listed on the letter. If your response provides information that justifies your entries, the IRS can reverse the amount due.
For more information regarding your CP105 notice, please visit the IRS website.