The second time was the charm for first responders searching for the cause of elevated carbon monoxide readings at a multi-unit home in Madison early Monday morning. Firefighters responded to a report of a carbon monoxide alarm sounding at the home in the 900 block of East Johnson Street around 2 a.m. A resident told firefighters the alarm had been sounding for roughly two hours; another resident reported mild symptoms possibly indicating carbon monoxide exposure, according to a Madison Fire Department incident report. Firefighters traced the highest concentration of carbon monoxide to a rear living area but were unable to identify a source after ruling out nearby vehicles, appliances and a fire pit.