Company officials said the device performed without problem during January's Sugar Bowl and other earlier events, but has been removed and will be replaced.
All systems at the Superdome are now working and the dome will host a major Mardi Gras event Saturday night, said Doug Thornton, an executive with SMG, the company that manages the stadium for the state.
The power failure at Sunday's big game cut lights to about half of the stadium for 34 minutes, halting play between the Baltimore Ravens and San Francisco 49ers.
The FBI had ruled out cyberterrorism as a cause.
Entergy's announcement came shortly before officials appeared before a committee of the City Council, which is the regulatory body for the company, to answer questions about the outage.
Entergy New Orleans CEO Charles Rice and Dennis Dawsey, an Entergy vice president for distribution, told the Council that SMG agrees the cause of the outage was a relay failure. Asked if the two corporations still plan to hire a third-party investigator, Rice said that possibility remains open.
Committee member Jackie Clarkson pressed for such an independent probe. "We've told the public we're going to have an outside investigation," she said.
"We'll work closely with SMG and if there is a need for a third-party investigation, we will do that," Rice said.
It remains unclear whether the problem with the relay was a design flaw or a manufacturing problem. Rice said Entergy is working with the manufacturer.
"I'm pleased that we were able to find the root cause," said SMG executive Doug Thornton.
Entergy and SMG had both upgraded lines and equipment in the months leading up to the Super Bowl. Rice said the new switching gear, with the faulty relay, was installed as part of a $4.2 million upgrade by Entergy, including the installation of a new power line dedicated solely to the stadium.
In a separate project, SMG replaced lines coming into the stadium after managers expressed concerns the Superdome might be vulnerable to a power failure like the one that struck Candlestick Park during a 49ers Monday Night Football game in 2011. That outage was blamed at least partly on a transformer explosion.
Thornton stressed Friday that the dome was drawing only about two-thirds of its power capacity Super Bowl night, and said typical NFL games in late August or September can draw a little more.
City officials had worried that the Super Bowl outage might harm New Orleans' chances of getting another NFL championship game.
But NFL Commissioner Roger Goodell downplayed that possibility after the outage, saying the NFL planned to keep New Orleans in its Super Bowl plans. Mayor Mitch Landrieu said the city intends to bid for the Super Bowl in 2018.