Those aircraft were taken out of service for what the airline called ''critical maintenance checks.''
Operations at O'Hare Airport appeared back to normal Wednesday. The monitors in United Airline's terminal at showed only one delay Wednesday morning and one flight undergoing maintenance.
There were no early-morning cancellations, and no passengers speaking with ABC7 Chicago had been delayed.
United Airlines has been performing emergency maintenance on all 96 of its 757 aircraft since Tuesday. The procedure is grounding those planes for up to an hour and a half.
Flights were canceled and delayed Tuesday after the airline discovered a problem in one plane.
"I think they're being proative. They didn't have to do it. It wasn't a mandatory thing. Any time it's maintenance, it's cause for a little concern, but it's not stopping us from going to Vegas today," said passenger Carol Hosler, who was about to board one of the 757s, which -- presumably-- has been retooled.
The airline took the voluntary maintenance action to comply with the federal government's airworthiness directive. The FAA did not ground the fleet.
Mechanics have been modifying the air data computer systems, which take air pressure and speed readings for the 'auto pilot' and other critical functions.
"I think it's a little alarming, actually. I heard it was a software problem. Just today, I think everybody is a little bit leery about getting in the air," said United passenger Ken Keefer.
"I heard the report this morning that they canceled some flights from last night. I looked online and saw mine was ok , and one flight was canceled, an inbound flight. Hopefully, I will be all right," passenger Akida Anderson said.
United Airlines says only domestic flights were affected by the grounded aircraft. However, as repairs and inspections were being made Tuesday, the airline canceled 15 flights and delayed dozens of others.
"It is an inconvenience in that my husband will be waiting for us. He is already down there for work, and I'm not sure how we're going to get there now," Mary Davison said Tuesday night.
United says it rebooked many flights on different planes and different airlines.
Some delays were possible Wednesday. Travelers are advised to check flight status, especially if the scheduled flight is on a 757.
The maintenance on the grounded 757 aircraft is scheduled to be completed Wednesday.