IRS Letter 12C asks you to send in specific documents, most often Form 8962 and Form 1095-A, so the IRS can finish processing a tax return that claimed the Premium Tax Credit. It's a request for missing information, not an audit, and responding by the deadline on your letter keeps your return and any refund moving.

Letter 12C is a request, not an audit. The IRS sends it when it needs more information to finish processing an individual tax return, most commonly because the Health Insurance Marketplace reported advance payments of the Premium Tax Credit on your behalf, and your return didn't include Form 8962 to reconcile those payments against what you actually qualified for.
If that's why you received it, the IRS needs two things back from you: a completed Form 8962 (Premium Tax Credit), and a copy of your Form 1095-A (Health Insurance Marketplace Statement). You have 20 days from the date on your letter to respond, and you should respond even if you disagree with what the letter says. Once the IRS receives your response, it processes your original return using that information, you don't file an amended return (Form 1040-X) for this.
Letter 12C is the one page in this cluster where there is no single fax number to publish, and IRS.gov says so directly: "Your Letter 0012C provides a fax number, if you want to send the information by fax. Fax your information to the fax number in the notice." The number is printed on your specific letter, not on any IRS.gov page, because it depends on which IRS processing center is handling your case.
Taxpayers have reported several different numbers on their own Letter 12C notices over time, including 855-204-5020, 855-309-9361, and 844-254-2835. These are shared here for reference only, they are not published by the IRS itself, they are not guaranteed to be current, and using the wrong one risks losing your fax and delaying your refund. Always fax to the exact number printed on the letter you received, not to a number from this list or anywhere else online.
Once you have the right number, Fax.Plus lets you send your Form 8962, Form 1095-A, and your letter's own cover page from a computer, phone, or email, with a timestamped delivery confirmation for your records, useful since the IRS itself does not confirm receipt of your fax.
Once you know which fax number your letter lists, gather your completed Form 8962, your Form 1095-A, and the letter's own cover page (so the IRS can match your fax to your case). Fax.Plus lets you sign and send all three straight from a computer, phone, or email to the exact number on your letter, with a timestamped delivery confirmation to keep for your own records, since the IRS won't send you one.
Your return is on hold until the IRS gets what it asked for, so responding is what gets your processing (and any refund) moving again. The IRS states it will issue any refund due about 6-8 weeks after receiving your response. If you don't respond, the IRS will adjust your return without your input, based only on what it already has, which for the Premium Tax Credit case generally means disallowing the credit rather than crediting it. Responding by the deadline, even if you disagree with the letter, keeps you in control of that outcome.
The IRS will adjust your return using only the information it already has, without your input, generally not in your favor. For the Premium Tax Credit case, that typically means the credit gets disallowed rather than reconciled in your favor. Responding within your letter's 20-day window is what keeps that decision in your hands.
Yes, mail is also an accepted way to respond, your letter will list a mailing address alongside the fax number. Faxing is generally faster since mail has to physically travel and get logged before processing starts, which matters when you're already working against a 20-day deadline.
There's no substitute copy to look up online, the fax number is specific to your letter and isn't published anywhere on IRS.gov. Contact the IRS directly (the general individual assistance line is 800-829-1040) to confirm your case details and the correct number, rather than guessing or reusing a number you find elsewhere, including on this page.
The IRS can't help with this one, it says directly that it cannot answer questions about your Form 1095-A, reissue a missing form, or issue a corrected one. Log in to your HealthCare.gov account, or your state Marketplace's account if you didn't use HealthCare.gov, to retrieve it, or contact that Marketplace directly.
No, and you shouldn't file one. The IRS specifically instructs taxpayers not to submit a Form 1040-X after receiving Letter 12C, it uses the Form 8962 and Form 1095-A you send back to finish processing your original return as filed, not to replace it.
An IRS letter asking for missing documents is stressful enough without also wondering if you're faxing it to the right place. Here's what else comes up around IRS notices, Premium Tax Credit reconciliation, and getting a stalled return moving again.
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