Short answer: You get a grace period before your coverage ends. If you have a Marketplace plan and use the premium tax credit, the grace period is usually three months, as long as you have already paid at least one full month’s premium that year. If you do not pay everything you owe by the end of it, your coverage can be canceled back to the first month you missed.
Pay your monthly premium directly to your insurance company, not the Marketplace, by the due date. If you fall behind, the insurer cannot end your coverage immediately. The premium payment grace period is usually 3 months if you have both a Marketplace plan with the premium tax credit and have already paid at least one full month’s premium during the year. Without the premium tax credit, your state sets the grace period.
The three-month clock starts the first month you missed, even if you pay later months on time. Federal rules provide a grace period of 3 consecutive months for an enrollee receiving advance premium tax credit. If you do not pay all owed premiums by the end, you can lose coverage back to the first unpaid month, and your insurer may not cover services during the second and third months. Losing coverage for non-payment does not open a Special Enrollment Period, so you would wait for Open Enrollment.