Boeing is targeting regulatory approval for the fixes in October, though the US Federal Aviation Administration has said it does not have a firm timeline to put the jets back in the air.
Meanwhile, airlines that had purchased the fuel-efficient MAX have had to cancel thousands of monthly flights as they scramble to meet demand with slimmer fleets, eating in to profit and hurting some growth plans.
American Air, with 24 MAX jets at the time of the grounding and dozens more on order, said it will cancel about 140 daily flights through December 3, more than the 115 daily cancellations it was making through the summer.
It had previously pulled its 737 MAX jets from its flight schedule through November 2.
Customers whose flights are affected by the cancellations will be contacted by American or their travel agent, the carrier said.
Among other US airlines that operate the MAX, Southwest Airlines Co has canceled flights into early January and United Airlines until December 19.
Reuters with AP