On the paper
List every estimated payment you made for the year, with its date and amount, and compare that list against the payments the notice says the IRS posted.
Check the year each payment was applied to, not just the total. The fourth-quarter payment made in January is the usual culprit, because a payment intended for the prior year and posted to the current one produces a shortage on one year and a surplus on the next. A CP23 for one year and a CP24 for the next year are frequently the same payment seen from both sides.
Find the date printed on your notice or letter. The paper notice and the current official IRS instructions control, even where this page differs.
CP23, when the change leaves a balance
The IRS page for CP23 says the IRS corrected the return because it found a difference between the amount of estimated tax payments on the return and the amount posted to the account, and that the correction left an amount owed.
It says to pay the amount owed by the date shown on the notice to avoid additional penalties and interest, and to contact the IRS at the number on the notice by the date indicated if you disagree. Neither instruction carries a number of days on the IRS page; both point to the paper.
A CP23 balance built from a payment the IRS cannot find is a different problem from a CP23 balance built from a payment you did not make. The first is resolved with a payment record. The second is resolved by paying or by arranging to.
CP24, when the change leaves a credit
CP24 comes from the same comparison and reaches the opposite result: the IRS posted more estimated tax than the return claimed, so the account carries a credit.
The IRS page for CP24 says that if you agree with the changes, no response is required, and that you should receive the remaining refund within 4 to 6 weeks as long as you do not owe other taxes or debts the IRS is required to collect. If you disagree, it says to contact the IRS at the number on the notice by the date indicated.
A credit is still worth checking. An unexplained surplus in one year often means a payment meant for another year landed here, and the year it was meant for may be running toward a balance-due notice of its own.
Why one page covers both
The two notices are one IRS process with two outcomes. The comparison is the same, the records that answer it are the same, and the reader with either notice in hand is doing the same work: reconstructing where each estimated payment went.
Keeping them together also keeps the cross-year picture visible, which is where most of these notices are actually resolved.
Where this notice comes from
The IRS posts estimated tax payments as they arrive and matches them against what the return claims when the return is processed. These notices are the output of that match.
The mismatch is usually a timing or a labeling problem. Payments made near a year boundary, payments made through a spouse's account, and prior-year overpayments elected to be applied forward are the three recurring sources.
Because the records that resolve either notice are your own bank and confirmation records, this is one of the few IRS notices most people can answer without anything from a third party.
General next steps
- Pull your payment records: bank debits, confirmation numbers from the IRS payment systems, and any cancelled checks.
- Match each one to a quarter and to a tax year.
- Use the contact instructions on the notice for anything that does not match, and keep a copy of what you send.
Timing
Neither IRS page states a period. Both refer to the date printed on the notice, CP23 for paying and for disagreeing, CP24 for disagreeing. The date printed on the notice controls; do not calculate a deadline from this page.
What CP23 and CP24 does not mean
Neither notice is an audit, and neither questions the income or the deductions on the return. The subject is payments and where they posted. A CP23 also does not mean a payment was lost; far more often it was applied to a year or a quarter other than the one the return claimed it for.
What this page cannot tell you
This page cannot see your IRS account or tell you which tax year a specific payment was posted to.
Official sources
If anything on this page differs from your notice or from current IRS instructions, follow the notice and the IRS.
Common questions
What is the difference between CP23 and CP24?
The comparison is identical. CP23 is the version where the corrected estimated tax payments leave a balance owed, and CP24 is the version where they leave a credit.
I made the payment. Why does the IRS say it did not receive it?
Usually it received it and posted it somewhere else: a different quarter, a different tax year, or a different taxpayer identification number. Your confirmation record, with the date and the year you designated, is what locates it.
Do I need to respond to a CP24 if I agree?
The IRS says no response is required when you agree with the changes, and that the remaining refund should arrive within 4 to 6 weeks if you owe nothing else it is required to collect.
Could a CP23 and a CP24 be about the same payment?
Yes, in adjacent years. A payment applied to the wrong year creates a shortage on one and a surplus on the other, and the two notices arrive separately.