While an audit liability arises in the year an AMT credit carryover is used, the problem usually is not with that year but an earlier year where the carryover first arose and then was faithfully transferred from year to year until it could be used up.
Review your old tax returns to find which year the carryover originated. The carryover is reported on Form 8801, AMT Credit Carryover. A quick way to tell if the carryover was included in a particular year is to see if there is an entry for Form 8801 on Line 54 of your Form 1040 (for 2018, Form 1040 Schedule 5 line 74).
The interview for prior year minimum tax credit asks for two numbers, one from Form 6251, Alternative Minimum Tax, and one from Form 8801, Credit for Prior Year Minimum Tax. When TurboTax was used the year before but there was no 8801 that year, TurboTax transfers the prior year AMT (F/6251 Ln/36) but leaves the F/8801 Ln/30 blank because there was no 8801 to pull a number from (see attachment).
At that point, sometimes people will make an entry in the blank field for the amount on Line 30 on the only AMT-related form in the prior year's return, the 6251.
Line 30 on the 8801 form is the amount of AMT credit to be carried over to the next year. Line 30 on Form 6251 is the AMT exemption amount, a large number.
By entering the amount from the wrong form, a carryover is recorded. A portion of that phantom carryover is taken as a credit each year since, with the remaining balance rolling over to the following year.
To fix the carryover for a current return you have not yet filed (or want to amend):
1. In TurboTax, go to Deductions & Credits
2. Scroll down to Estimates and Other Taxes Paid (about halfway down the page).
3. Click on Start (or Update) for Credit for AMT Paid in Prior Year.
3. The next screen will ask if you paid AMT in the past -- click Yes
4. On the Credit for Prior Year AMT screen, he erroneous carryover amount will be prepopulated on the line for the Form 8801 carryover. Enter a zero to prevent the credit from being calculated for the current year and carrying forward to future years.
5. Click Continue to save the entry. You should see a message that "you don't qualify for the credit because you didn't have a 2017 minimum tax credit to carry forward."
For more information, look at:
-- Can I claim the AMT (Alternative Minimum Tax) credit?
https://ttlc.intuit.com/community/credits-and-deductions/help/can-i-claim-the-amt-alternative-minimu...
-- What is the AMT credit?
https://ttlc.intuit.com/questions/4396732
-- What is the Alternative Minimum Tax (AMT)?
https://ttlc.intuit.com/questions/1899184
