After shutting down three times in three days, Kelly’s Restaurant in Quincy is back open.

The establishment reopened Friday morning after shutting down twice Tuesday because of high carbon monoxide levels in the business, and then again Thursday afternoon to make sure the issues had been resolved.

It was determined that a stove and a rooftop furnace were the causes of the dangerously high CO levels.

About a dozen employees and customers were treated for symptoms consistent with exposure to carbon monoxide.