ORLANDO, Fla. -- A Southwest Airlines Boeing 737 has made an emergency landing at Orlando International Airport.

Flight number 3238 from St. Louis landed at OIA Tuesday afternoon with 128 passengers on board.

No injuries were reported.

During the flight, there was an indication of fire in the auxiliary power unit, OIA officials said.

"The plane came to a dead stop on the runway and then we saw the fire trucks out there, and they alerted us to what was going on," passenger Larry Amis said.

Passengers and crew were evacuated via stairs on the runway and bussed to the terminal.

"It was a little scary. It was exciting. But nobody was crazy or anything. Everybody was really cool and calm," a passenger said.

Fire officials inspected the plane and did not find any evidence of a fire. The pilot announced to the passengers after landing that the problem was a malfunctioning sensor.

"There was no indication of smoke or smell of smoke or anything. I guess he suspected a faulty light, but he took all the normal precautions," a passenger said.

Many passengers said they were aware that another Southwest flight was grounded on Monday after a hole was torn into the ceiling of a plane mid-flight.

Southwest Airlines inspected nearly 200 planes after a foot-long hole opened in the cabin of an identical jet, forcing the aircraft to make an emergency landing in West Virginia.

Passengers aboard a Nashville-to-Baltimore flight Monday said they could see through the hole above them, just in front of the plane's tail. The cabin lost pressure, but no one was injured on the flight, which carried 126 passengers and five crew members.

Passenger Michael Cunningham told NBC's "Today" show Tuesday that he had dozed off in his seat in mid-cabin when he was awakened by "the loudest roar I'd ever heard."

Cunningham said the hole was above his seat. People stayed calm and put on the oxygen masks that dropped from the ceiling.

"After we landed in Charleston, the pilot came out and looked up through the hole, and everybody applauded, shook his hand, a couple of people gave him hugs," he said.

Southwest said it was unclear what caused the damage.

The National Transportation Safety Board sent an investigator to the scene to interview the crew and examine maintenance and inspection records, but could take months to find a cause, said agency spokesman Keith Holloway.

The incident occurred just four months after Southwest agreed to pay $7.5 million to settle charges that it operated planes that had missed required safety inspections for cracks in the fuselage.

Southwest spokeswoman Marilee McInnis said the airline inspected its Boeing 737-300-series jets overnight at hangars around the country and discovered no cracks in any of the others.

"It was a walk-around visual inspection just to check for structural integrity," McInnis said.

Southwest flies 181 of the 137-seat 737-300. All its 544 jets are various models of the Boeing 737.

The planes that were inspected overnight were put back into routine service Tuesday morning, while the airliner that landed in West Virginia stayed there. Representatives from aircraft manufacturer Boeing were helping to determine cause of the hole, McInnis said.

Southwest operated a normal schedule of flights -- about 3,300 per day -- with no cancelations or delays through midmorning, McInnis said.

The hobbled airliner was placed in service during the 1990s and went through "routine maintenance" this month, McInnis said.

In 1988, cracks caused part of the roof of an Aloha Airlines Boeing 737 to peel open while the jet flew from Hilo to Honolulu. A flight attendant was sucked out of the plane and plunged to her death, and dozens of passengers were injured. The incident led to tougher rules for inspecting fuselages.

In March, Southwest agreed to pay a $7.5 million civil penalty imposed by the Federal Aviation Administration for operating nearly 60,000 flights in 2007 on planes that had not undergone required inspections for cracks in the fuselage. About 1,450 flights took place after the FAA had notified Southwest of the missed inspections.

The settlement with the FAA also required Southwest to increase oversight at companies to which Southwest outsources maintenance work, and to give FAA inspectors more access to information used in tracking maintenance activities.

Dallas-based Southwest Airlines Co. carries more than 100 million U.S. passengers a year, more than any other airline.