A car left running in a garage caused a brief carbon monoxide scare Wednesday evening, fire officials said.

The Madison Fire Department said an east side woman wasn’t sure why her carbon monoxide detector was sounding at 6:30 p.m., but she called 911 and evacuated the building.

Firefighters met her outside before bringing their air monitor into the residence on the 3700 block of Design Pass. Their meters detected carbon monoxide at a concentration of 300 parts per million. MG&E also responded to the residence and assisted in identifying the source of the odorless, colorless gas.

As firefighters ventilated the residence, MG&E detected carbon monoxide readings just outside the next unit. The neighbor there allowed firefighters inside to take a closer look. As they opened the garage access door, MG&E’s air monitor readings spiked to 3,200 parts per million. The residence was immediately evacuated and ventilated until air quality returned to normal and safe levels.

Firefighters and MG&E identified the carbon monoxide source to be a vehicle that had been running in the garage unbeknownst to its owner.

Crews checked the other neighboring unit to see if it was affected by the carbon monoxide buildup. They did find readings at 5 parts per million and had the resident open windows to release the low levels of CO.