Illinois Form ST-556-X: Correcting, Filing, and Paying Amended Sales Tax

Illinois Form ST-556-X is the Amended Sales Tax Transaction Return that registered retailers and leasing companies use to correct a previously filed ST-556 covering the sale of a vehicle, watercraft, aircraft, trailer, or manufactured home titled or registered by an Illinois state agency.¹ You file it whether the original understated the tax, overstated it, or simply carried a wrong VIN or purchaser name.

When You Need to File

The Illinois Department of Revenue lists four situations that call for an ST-556-X:¹

You need to pay additional tax because the original return underreported the amount owed.
You overpaid and want a credit. Do not file an ST-556-X for overpayments of less than $1, and do not apply any credit until the Department notifies you it has been approved.
The Department sent you a bill or notice about a discrepancy on the original return.
You need to fix non-financial details like the purchaser’s name, VIN, or item description without changing the tax.

Non-financial fixes matter more than they look. A wrong VIN or misspelled name on the original return can block title registration at the Secretary of State, so catching those early spares the buyer trouble at the counter.

Paper or Electronic

How you filed the original ST-556 dictates how you file the amendment. Originals filed through MyTax Illinois at mytax.illinois.gov must be amended electronically through the same portal. Originals filed through an approved service provider in the Electronic Registration and Title (ERT) program, or on a preprinted paper form, must be amended on paper.¹

Mail paper returns to:

ILLINOIS DEPARTMENT OF REVENUE
PO BOX 19034
SPRINGFIELD IL 62794-9034²

Filling Out the Form

Everyone completes Steps 1, 2, and 4. Step 3 applies only when the amendment changes financial figures such as the selling price, trade-in allowance, or tax due.³

Step 1: Identify the Return

Line 1 asks for the tax return number from the original ST-556. That number links the amendment to the correct filing. Lines 2 through 4 ask for your Illinois account ID, business name exactly as it appeared on the original, and a daytime phone number.

Step 2: Mark Your Reason

Choose one of the numbered reasons, 1 through 8. Several require supporting details on the form itself:

Drive-away permit or out-of-state plates (Line 4a): enter the permit number or license plate number and the two-letter state abbreviation.
Sale to a dealer (Line 4b): enter the purchasing dealer’s account ID. For out-of-state dealers, write “Out-of-State Dealer.”
Tax-exempt organization (Line 4c): enter the organization’s active Illinois Sales Tax Exemption “E” number.
Certificate of authority (Line 4d): enter the certificate of authority number.
Other reason (Line 8): mark this only if reasons 1 through 7 do not apply, and write an explanation on the lines provided.

Step 3: Correct the Financial Information

Step 3 runs in two columns. Column A takes the figures from your most recent return; Column B takes the corrected figures. Where a line has no change, repeat the Column A number in Column B. Round to the nearest whole dollar.²

The corrected selling price on Line 1 should include accessories, federal excise taxes, freight, labor, dealer preparation charges, documentary fees, and any rebates or incentives for which you as the dealer received reimbursement. Line 4 takes the appropriate combined tax rate for your area. Line 7a applies a 0.5 percent calculation to your Line 3 figure.³

Step 4: Sign and Date

Everyone completes Step 4 regardless of the type of correction. The form must be signed by the owner, officer, or another person authorized to sign the original return.

If You Owe More Tax

Send payment with the filing. For paper returns, make the check or money order payable to the Illinois Department of Revenue and mail it with the form to the Springfield address above. Interest on underpayments accrues daily at a rate tied to the federal underpayment rate under Internal Revenue Code Section 6621, and the Department recalculates that rate every January 1 and July 1.⁴ The longer you wait, the more you owe.

If You Overpaid

An ST-556-X that shows an overpayment produces a credit memorandum, not a check. You can apply that credit to any future tax liability under the same tax act, including penalties and interest.⁵ Do not deduct the overpayment from a future return on your own. Wait for the Department to issue the credit memorandum before applying any offset.

To get cash instead of a credit, file a separate Form ST-6, “Claim for Sales and Use Tax Overpayment / Request for Action on a Credit Memorandum.” Select item 5 and enter the total overpayment to convert an overpayment to cash, or item 8 and enter the balance to convert an already-issued credit memorandum to cash. Mail Form ST-6 to:⁶

SALES TAX PROCESSING DIVISION
ILLINOIS DEPARTMENT OF REVENUE
PO BOX 19013
SPRINGFIELD IL 62794-9013

Cash refunds are not guaranteed. The Department pays them from a dedicated appropriation, and when that fund looks insufficient to cover approved claims, refunds go out only in hardship cases where the taxpayer cannot use a credit memorandum.⁵

There is also a filing deadline for credit claims. No tax, penalty, or interest paid more than three years before the applicable January 1 or July 1 filing window can be credited or refunded. Claims filed between January 1 and June 30 cover the current year plus the previous 36 months; claims filed on or after July 1 cover the current year plus the previous 30 months. The postmark controls timeliness. If you and the Department have agreed in writing to extend the time for issuing a notice of tax liability, your claim window extends to match.⁷

Late-Payment Penalties

Voluntarily filing an ST-556-X to pay additional tax is cheaper than waiting for the Department to find the error. The late-payment penalty escalates:⁴

1 to 30 days late: 2 percent of the unpaid tax.
31 or more days late: 10 percent of the unpaid tax.
Discovered during an audit: 15 percent of any amount not paid before the audit or investigation begins.
Not paid within 30 days of an audit-prepared amended return: 20 percent.

Records to Keep

Hold onto every document that supports both the original ST-556 and the ST-556-X: the bill of sale, manufacturer’s statement of origin, corrected invoices, and any signed affidavits attached to the amendment. Illinois requires you to keep these records for three and a half years after filing the original or amended return.⁸ If the Department has issued a Notice of Tax Liability or Final Notice of Tax Due for that period, keep the records until the liability is finalized or discharged, even if that runs past the normal window.

1
Illinois Department of Revenue. ST-556-X Amended Sales Tax Transaction Return Instructions
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Illinois Department of Revenue. ST-556-X Amended Sales Tax Transaction Return
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Illinois Department of Revenue. ST-556-X Amended Sales Tax Transaction Return Instructions
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Illinois Department of Revenue. Pub-103, Penalties and Interest for Illinois Taxes
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Illinois General Assembly. 86 Illinois Administrative Code 470.140 – Claims to Recover Erroneously Paid Tax
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Illinois Department of Revenue. Claim for Sales and Use Tax Overpayment / Request for Action on a Credit Memorandum
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Illinois Department of Revenue. When Is Form ST-1-X Due?
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Illinois Department of Revenue. Pub-113, Keeping Complete and Accurate Records