The odor was first noticed Saturday afternoon in Haverford Township, Marple Township, Radnor and Upper Darby - all in Delaware County.
Several residents in Chester and Montgomery counties also noticed the odor in Paoli, Lower Merion, Plymouth-Whitemarsh, Jenkintown and Abington.
PECO has also gotten calls from Warminster in Lower Bucks County.
A spokesperson says they have gotten over 900 calls but have so far found no problems in their system.
PECO officials are still trying to determine the source of the odor.
Any resident that smells a gas odor is urged to contact their help line at 1-800-841-4141.