Boeing has decided to bite the bullet and won’t seek an exemption after the Federal Aviation Administration (FAA) rejected the jetmaker’s argument that the electrical wiring on the 737 MAX is compliant with regulations for safe wire separation.

Its engineers are preparing to go ahead and rewire all the jets that Boeing has stored while the FAA decides how to handle the planes already delivered. In total there are about 800 jets in the currently grounded MAX fleet.

“This work should take approximately five days per airplane to include the required modification work and associated testing,” a Boeing official said Thursday.

A person familiar with the plan said, “We can start going in to the first airplane as early as this week and see exactly how to do it, and figure out how to get it done right. Those first few planes will take longer.”

The person said Boeing doesn’t expect the work to change its anticipated schedule for clearing the plane to fly again by midyear.

“A lot of this can be done at the same time as the other work on taking the planes out of storage,” he said. “That process takes a week or two itself, running all the checks — on fluids, tires, fuel tanks, avionics, engines — to make sure the plane is airworthy and then installing the new software” to fix the flight control system.

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Boeing’s failure to meet current regulations on the wiring is just one of many issues that surfaced after the jet was grounded following the two fatal crashes in Indonesia and Ethiopia that killed 346 people.

While Boeing initially intended only a software fix for the flight control system implicated in the crashes —Maneuvering Characteristics Augmentation System — the intense scrutiny of the MAX systems has revealed a litany of other problems that have extended the grounding to a year at this point and continue to add to Boeing’s costs.

The problem with the wiring is that low-voltage wires controlling the motor that swivels the jet’s horizontal tail, or stabilizer, are co-routed with and insufficiently separated from a hot power wire. The concern is that a short circuit could command the stabilizer to move without pilot input, which would pitch the aircraft’s nose either up or down.

This is considered a remote risk. The prior Boeing 737 NG model has racked up 205 million flight hours with that same wiring and no hot shorts.

Nevertheless, the current regulation requiring physical separation of these wires was introduced in 2009 following study of two fatal crashes of other jet models: TWA 800 in 1996, in which an electrical short is believed to have caused a spark in the fuel tank and an explosion; and Swissair 111 in 1998, when an electrical short caused a fire in the cockpit.

The 737 NG model was certified in 1997 and so the regulation doesn’t apply. When Boeing certified the MAX in 2017, its engineers somehow missed that the wiring needed to be changed to meet the latest standard, and the FAA failed to detect Boeing’s miss.

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Boeing realized the mistake during its intensive review of all the MAX systems after the jet was grounded following the two MAX crashes.

It then argued to the FAA that the safe history of the 737 NG meant that the wiring didn’t need to be modified and could be considered compliant.

The FAA didn’t buy it. On Monday, the safety agency issued a statement insisting that Boeing “must demonstrate compliance with all certification standards.”

Boeing could have come back with a further argument that, although not compliant, the wiring wasn’t critical for safety, requesting an exemption. But the company decided that was unlikely to succeed.

The modification Boeing must now perform will require removal of a number of avionics boxes and associated shelving from the electrical equipment bays in the lower half of the fuselage underneath the passenger cabin floor, the Boeing official said.

The wiring that must be modified is in 10 to 12 locations, mostly in those electrical equipment bays, though one of the wire bundles that needs to be modified is in the tail cone of the airplane.

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“We will cap and stow a wire and reroute a new wire separated from the power wire,” said the person familiar with the plan.

Boeing has not yet informed the FAA of the details of its proposed wiring fix.

Once it does so, because the danger is remote and the lack of separation is not considered an imminent safety of flight issue, the FAA will have the option to issue an airworthiness directive requiring the work be done within a certain time period.

However, airlines may insist the wiring be modified before any of their grounded MAXs fly again.

That means getting the grounded planes back in the air could be slowed. But Boeing could still meet its midyear schedule for a MAX return to service by first delivering the airplanes that roll out of the factory once FAA clearance is given.

The wiring issue is the last remaining barrier to the FAA conducting a certification flight on the MAX, which would be followed by Boeing turning in the final documentation needed for recertification of the aircraft, according to a second person familiar with the details of the process.

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In parallel, pilot training requirements could be pinned down by the Joint Operations Evaluation Board — composed of pilots from U.S. and international airlines operating the 737 — and the FAA’s own Flight Standardization Board.

The wiring is just one of three new issues that arose recently with the MAX.

Last month, the FAA issued an airworthiness directive requiring all 737 MAXs to be inspected for a manufacturing defect that leaves the engine pods, called nacelles, vulnerable to a lightning strike.

Mechanics polishing the carbon composite nacelles at the end of the production process ground away some underlying layers of metal foil in the upper part of the pod that are necessary for lightning protection.

Also in February, Boeing discovered debris that mechanics left inside the wing fuel tanks of some undelivered 737 MAXs during the aircraft assembly process.

Subsequent checks on the undelivered airplanes Boeing has parked found debris — including left-behind tools and rags — in about half the airplanes, the person familiar with the details said.

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All those nacelle panels will have to be inspected and fixed and all the fuel tanks will have to be inspected and cleared of debris before any of those already-built planes fly again.

With the airline industry reeling from the dramatic global impact of the novel coronavirus, on top of the 737 MAX grounding, Boeing’s shares fell 18% Thursday to $154.84 at the close and continued falling in after-hours trading.

A year ago, days before the second crash of Ethiopian Airlines Flight 302, Boeing’s stock hit an all-time high of $446.