Officials said Tuesday that letters are going out to about 310,000 people whose citizenship and information documentation doesn't match what the government has on file.
These consumers will need to upload their documentation to HealthCare.gov by Sept. 5, or mail it in.
Otherwise their coverage will end Sept. 30.
Of the 8 million people who signed up for private coverage through President Barack Obama's law, more than 2 million at one point had discrepancies of some sort that affected their eligibility.
That number has been greatly reduced — but the remaining cases are proving difficult to clear up.