WASHINGTON - President Obama's push to cover America's uninsured faces another big test Monday.

This time, it's not only how the website functions, but also how well the Affordable Care Act works for millions who are starting to count on it.

Monday is the deadline for new customers to pick a health plan that will take effect Jan. 1, and for current enrollees to make changes that could reduce premium increases ahead of the new year.

HealthCare.gov and state insurance websites are preparing for heavy online traffic before the deadline, midnight Pacific time, which gives consumers in the East three hours into Tuesday to enroll.

Wait times at the federal call center started creeping up last week, mainly due to a surge of current customers with questions about their coverage for next year. Many will face higher premiums, although they could ease the hit by shopping online for a better deal. Counselors reported hold times of 20 minutes or longer.

About 6.7 million people now have coverage under Obama's signature law, which offers subsidized private insurance. The administration wants to increase that to 9.1 million in 2015.

People can no longer be turned down because of health problems, but picking insurance still is daunting for many consumers. They also have to navigate the process of applying for or updating federal subsidies, which can be complex for certain people.

Consumers "understand it's complicated, but they appreciate the ability to get health insurance," said Elizabeth Colvin of Foundation Communities, an Austin, Texas, nonprofit that helps sign up low-income residents. "People who haven't gone through the process don't understand how complicated it is."

Last year's open enrollment season turned into a race to salvage the reputation of the White House by fixing numerous technical bugs that crippled HealthCare.gov from its first day. With the website now working fairly well, sign-up season this year is a test of whether the program itself is practical for the people it is intended to serve.

New wrinkles keep popping up, even with seemingly simple features of the Affordable Care Act.

Most current customers who do nothing, for example, will be automatically renewed Jan. 1 in their plan. It looks like that is what a majority intends to do.

While that may sound straightforward, it's not.

By staying in their current plans, people can get locked into a premium increase and miss out on lower-price plans for 2015. They also will keep their 2014 subsidies, which may be less than what they legally would be entitled to for next year.

Monday is not the last chance for current customers: Open enrollment doesn't end until Feb. 15.