Oil resumed flowing Monday afternoon through the Keystone oil pipeline that carries about 590,000 barrels of crude per day from Canada to facilities in the Midwest.
TransCanada had shut down the 2,100-mile pipeline Wednesday after tests showed possible safety issues. Company spokesman Shawn Howard said in an email that there were no leaks and "the integrity of the pipeline system is sound."
The system will be operated at a slightly reduced pressure for about 24 hours so additional testing can be completed.
"Once the pipeline system is operating at full pressure, we will be curtailing October volumes and will return to contractual delivery levels in November," Howard said. "We will be having direct discussions with our customers regarding the impact this will have on their deliveries to us."
The potential problems were detected in a section of the line between Missouri and Illinois. TransCanada had planned to restart the pipeline Saturday, but bad weather complicated efforts to move equipment into the area so workers could excavate the pipeline for inspection.
"It was critical we moved forward safely," Howard said of the logistical issues.
The temporary closure isn't expected to affect refinery production or retail gas prices because crude supplies are plentiful across the U.S.
The shutdown comes amid delays over TransCanada's plans to build a $7 billion pipeline called the Keystone XL that would transport heavy tar-sands crude oil from Canada to Texas' Gulf Coast refineries.
Opponents warn the new pipeline would be carrying heavy, acidic crude oil that could more easily corrode a metal pipe and lead to a spill. TransCanada says its pipeline would be the safest ever built.