The CP 161 is not a math error notice. It shows the underpaid tax according to the IRS records. CP 161 shows the tax you reported on the return, the payments the IRS applied, and the resulting underpayment the IRS has on record.

This notice is a reconciliation issue and the starting point is generally to compare the payments made from your records to the payments in the IRS records. If the payments you made reconcile with the payments recorded by the IRS, pay the balance due by the due date on the notice. If you cannot pay the balance in full, contact the IRS to possibly setup an installment agreement. If one of your payments made is missing from the IRS listing, contact the IRS with documentation of the payment - cancelled check, direct debit information, etc.

Sections of the Notice:

• Header Section
  This area contains the Notice Number, the notice date, your Social Security Number and the IRS contact number.
• Body Section
  The body contains tables showing the difference between the IRS records of payments made, the total amount claimed per the tax return, and the resulting underpayment. If your records indicate the same payments and you agree with the IRS, submit a payment for the unpaid balance.
• Response Section
  The notice may have a coupon at the bottom (or end) of the notice to include with your payment.
  If your records do not agree with the IRS records you should contact the IRS at the address or phone number in the header section.

• PDF Pages 1-4