At issue is Form 1095-A, which health-law exchanges must send to individuals and families showing how much money they received in 2014 from the federal government to subsidize their health insurance premiums.
Covered California said it sent incorrect information on some forms because its customer data didn't match what health plans had on file.
For instance, there may have been a discrepancy for the person's length of coverage in 2014 and amount of subsidy received.
Amy Palmer, an exchange spokeswoman, said the agency is reconciling that information and sending revised forms to the affected customers by later this month.
She said customers will also be notified by email when the updated forms are available in their online account.
Overall, Covered California sent tax forms to more than 800,000 households statewide.
The accurate information is important for people when filing their 2014 taxes.
Consumers may have to repay some portion of that government assistance as part of their tax return if their income was higher than what they estimated during enrollment.
Meanwhile, the exchange said it's seeing strong consumer interest ahead of Sunday's deadline for open enrollment under the Affordable Care Act.
Thursday, Covered California gave applicants until Feb. 20 to finish signing up if they start the process by Sunday.
After this weekend, people will have to contact the exchange's call center or meet with an enrollment counselor, insurance agent, county health worker or health plan representative to complete the process.
Covered California has 1.3 million people enrolled thus far, counting renewals and new signups. It set a goal of 1.7 million for this open enrollment.