Chopper 6 was over the scene in the 500 block of Delancey Street, where about 20 houses were evacuated.
PECO, PGW and fire crews showed up at 6 a.m., looking for a source.
The Philadelphia Fire Department took carbon monoxide readings which prompted the evacuations. Once the carbon monoxide levels dissipated at around 7 a.m., most of the residents were let back into their homes.
PECO told Action News that it started as an issue with underground cables. This caused a strong odor in the area homes. PECO has since de-energized the area and crews remain on scene to make the repairs.
No injuries have been reported.