CHICAGO (CBS) — Engineers worked all weekend to install new carbon monoxide detectors at Prussing elementary school so students could return to class on Monday.

That leak sent 81 students and staffers from Prussing to the hospital suffering from CO exposure.

All are doing well, CBS 2’s Mike Puccinelli reports, and the school opened at its regular time Monday.

But there will be two engineers on hand along with the facility manager to make sure that the boiler is working properly.

A faulty valve in that boiler caused large amounts of carbon monoxide to leak Friday sickening the students. To make matters worse, the nearest CO detector wasn’t working.

School officials are trying to determine why the detectors didn’t go off.

“We know they detected the carbon monoxide, but we’re looking into whether we had the right technology and whether they were operating properly,” CPS CEO Forrest Claypool said Monday.

There were no detectors in the main building where students and staff are. CPS policy only requires CO detectors in boiler rooms or in school kitchens that use gas. Prussing’s kitchen is electric.

Ward 45 Ald. John Arena says principals need to be aggressive about putting CO detectors throughout school buildings “to make sure this situation isn’t replicated someplace else.”

Daniel Coss was one of the students who was rushed to the hospital.

“I couldn’t breathe and I had a headache,” he said.

Daniel’s father first received a notification, saying there was a problem at the school. Then a second notification was more alarming–that Daniel had been sent to the hospital.

At a Monday night meeting of the Local School Council, Richard Munoz was among the parents demanding answers after his 10-year-old son Jimmy was rushed to the hospital Friday.

“I just don’t understand how it could get to that level,” he tells CBS 2’s Jim Williams.

There are now nine working carbon monoxide in the building. But installing more detectors wasn’t enough to satisfy some parents. They said the boiler needs to be replaced.

LSC members said they’ve asked for a new boiler for years.