Aircraft manufacturing giant Boeing and US federal regulators said they have identified a new software problem on the 737 MAX, further delaying the process of returning the troubled jet to service.

The new issue involves software that is separate from changes to the aircraft’s faulty flight-control system, called the Maneuvering Characteristics Augmentation System (MCAS), Efe news quoted informed sources as saying on Wednesday.

The software system in question, though distinct from MCAS, is related to an emergency procedure that can be used by pilots to address MCAS malfunctions, the sources said.

The plane maker disclosed in a securities filing on Wednesday the US Federal Aviation Administration’s (FAA) request to address the new problem, which it said wouldn’t be covered by planned changes to the MCAS system.

Boeing said it agreed with the FAA’s decision and is working on the required software fix.

The software issue involves an emergency procedure that would be used to counteract MCAS if it malfunctions, erroneously pushing the plane’s nose down.

The FAA identified the problem last week during simulator tests, according to the sources, after an agency test pilot determined that the procedure took more time than was acceptable to execute.

The new problem is related to software that was original to the aircraft, not revisions in conjunction with changes to MCAS that were made after two fatal crashes of 737 MAX jets, according to one of the people familiar with the matter.

Boeing believes it can resolve the issue with a software tweak, these people said. Otherwise, they added, there is a chance that the company would have to replace a computer chip in all 500 MAX planes that have been delivered or built, a process that would add significant delays.

Certification flight tests, an important step before regulators around the world allow the MAX to fly again, had been expected to begin as soon as last week, according to people familiar with the details.

“Boeing will not offer the 737 MAX for certification by the FAA until we have satisfied all requirements for certification of the MAX and it’s safe return to service,” the company said in its filing.

An FAA spokesman said the agency is following a thorough process and has no timeline for allowing the 737 MAX to return to service. “The FAA recently found a potential risk that Boeing must mitigate,” he said.

A total of 346 people died in two 737 MAX crashes in Indonesia and Ethiopia that took place in a span of five months. Regulators around the globe grounded the aircraft after the Ethiopian crash in March.

United Continental on Wednesday became the latest US carrier to strike the MAX from its schedule until September 3, following similar moves by American Airlines and Southwest Airlines.

The airline added it was trying to minimize disruptions by swapping out aircraft and using bigger planes in lieu of the MAX, but it will still have to cancel some 1,900 flights in August, or 60 a day.

Wednesday’s developments illustrate how unexpected complications can upset months of painstaking preparations to get the MAX fleet back in the air.

Boeing originally submitted an MCAS software fix to the FAA in January, and since then the Chicago plane maker has been on a roller-coaster ride of shifting timetables and deadlines for getting the green light from the FAA.

It isn’t clear how long this latest delay might last.