It isn't what investors want to hear.

Boeing (ticker: BA) has run into a new problem with its 737 MAX jet. The issue will test investors nerves in coming weeks, and raise more questions about the company's ability to increase production in 2023.

Thursday, the aircraft maker said it has halted MAX deliveries after the discovery of a parts compliance problem from a supplier.

Boeing stock was down 5.8% in premarket trading Friday at $201.15. Shares gained 0.6% in regular trading on Thursday while the S&P 500 and Dow Jones Industrial Average rose 1.3% and 1.1%, respectively.

"A supplier has notified us that a nonstandard manufacturing process was used during the installation of two fittings in the aft fuselage section of certain [737 MAX] airplanes, creating the potential for a nonconformance to required specifications," said a Boeing spokeswoman in an emailed statement. "This is not an immediate safety of flight issue and the in-service fleet can continue operating safely. However, the issue will likely affect a significant number of undelivered 737 MAX airplanes, both in production and in storage."

Four MAX models are impacted: The 737-7, 737-8, 737-8-200 and the P8 military jet based on a commercial aircraft platform. How much of total MAX demand or backlog that represents isn't clear. It is a significant amount of MAX demand, according to the airplane maker. Boeing added that no nonconforming planes will be delivered and that it has notified the Federal Aviation Administration and it is working to inspect and replace nonconforming parts.

Shares of Spirit AeroSystems (SPR) were down 14.6% in premarket trading at $30.40. The company said in a statement that it had notified Boeing that it had found a quality problem on the aft section of certain models of the 737 fuselage it builds. It said it has systems in place to address production issues and that is following those processes.

Investors are particularly attuned to MAX issues because the jet was grounded worldwide between March 2019 and November 2020 following two deadly crashes inside of five months.

Boeing stock was north of $440 a share before the second crash, and before Covid-19. It closed on Thursday at $213.59.

The MAX has been flying safely for years since its reintroduction to commercial fleets, but Boeing's ability to boost production is a significant issue for the stock in 2023. Wall Street expects Boeing to deliver about 570 jets in 2023, up from 480 in 2022. Boeing delivered 806 jets in 2018, the year before the MAX issue and a couple of years before Covid-19 decimated demand for air travel.

Delivery pauses don't help with investor confidence regarding the 570 delivery mark. They also don't help with investor confidence about Boeing's recovery from MAX and pandemic-induced lows.

"Much depends on how quickly this problem can be dealt with," says AeroDynamic Advisory's Richard Aboulafia . "That will tell us a lot about Boeing's ability to work with regulators and suppliers on a solution. Right now it's too soon to tell."

The call for 570 deliveries is a Wall Street projection and includes single and twin-aisle jets. Boeing has told investors to expect 400 to 450 MAX deliveries for 2023. That will require getting the MAX production rate from about 31 a month to 35-plus a month. Wolfe Research analyst Myles Walton wrote Friday he is cautiously optimistic that can still be hit.

That will require a quick resolution to the current issue as well as dealing effectively with supply-chain issues persisting in the aerospace industry. Those aren't hitting only Boeing.

"Airbus had to cut its delivery target twice last year due to supply chain," said Vertical Research Partners analyst Rob Stallard. "It's still a struggle."

He rates Boeing shares at Hold, adding that while the continuing recovery in global air travel is a positive for the shares, continued execution issues will keep a lid on investor sentiment.

Boeing investors will hear more about MAX production and delays when the company reports first-quarter numbers on April 26. Boeing delivered 130 jets in the first quarter, better than the 120 Wall Street was looking for.

The first quarter of the year is seasonally weak for Boeing deliveries.