Technicians worked around the clock to meet the midnight deadline but the site was due to go offline for maintenance early on Sunday.
The test will probably come as people return from Thanksgiving holidays.
Problems with the website have led to a drop in President Barack Obama’s approval ratings.
White House officials said the HealthCare.gov site was working well on Saturday after overnight hardware upgrades to boost its capacity.
Software fixes to improve speed and reduce errors were planned for overnight Saturday into Sunday morning.
An update on the website’s progress is due to be released later on Sunday by Jeff Zients, the site’s chief troubleshooter.
HealthCare.gov is a key element of Barack Obama’s flagship 2010 Patient Protection and Affordable Care Act – also known as ObamaCare – which aims to provide affordable health insurance to the estimated 15% of US citizens who lack it.
But since its October 1st launch, the site has been plagued by errors, outages and slow speeds.
Despite the setbacks, Barack Obama has described his healthcare reform law as a legacy to be proud of.
In an interview with ABC News, Barack Obama said he still believed his Affordable Care Act would make good on his manifesto promise to deliver affordable health care to Americans.
ObamaCare has been strongly criticized by Republican party politicians and many private health providers, who say it is too expensive and an unwarranted intrusion into the affairs of private businesses and individuals.
Their opposition intensified after the troubled launch of HealthCare.gov in October.
The website, which services 36 states, signed up just 27,000 people in the first month while the 14 states that run their own websites enrolled 79,000.
The total of around 106,000 was far off the administration’s estimate that nearly 500,000 people would enroll within the first month.
The White House promised a “smooth experience” for the “vast majority” of website users by the end of November.
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