New York(CNN Business) Boeing found debris in the fuel tanks of several 737 Max jets that have been sitting in storage. It's the latest problem Boeing faces as it tries to get the 737 Max back in the air.

In a note to Boeing employees, the company said that foreign object debris, referred to as FOD, "is absolutely unacceptable. One escape is one too many. With your help and focus, we will eliminate FOD from our production system." The note was from Mark Jenks, vice president and general manager of the 737 program and the Renton, Washington, factory.

"During these challenging times, our customers and the flying public are counting on us to do our best work each and every day. That's why we're taking action," Jenks said in his note.

Boeing said it will address the problem by updating instructions and required checklists for employees, along with additional verifications including inspections and audits.

The Boeing 737 Max has been grounded since March following two fatal crashes that killed 346 people. Boeing has been trying to gain approval for a fix to its a safety system's software that is believed to be the cause of those crashes. The fix has taken much longer than expected to complete, but Boeing expects to get approval for the plane to fly passengers once again by the middle of this year.

Boeing continued to build the jets at a reduced pace until it temporarily halted production earlier this month. Nearly 400 of the completed jets are parked and awaiting delivery to its airline customers.

The note from Boeing did not say how many planes had debris discovered in the fuel tanks.

Debris on a completed aircraft is a serious quality problem that can pose significant safety threats on planes, including short circuits and fires. It is a problem that has dogged Boeing in the past.

The US Air Force temporarily halted deliveries of the KC-46 Pegasus tanker aircraft twice last year after debris was found in closed compartments of the aircraft.

There were problems with debris on the 787 Dreamliners built at a plant in South Carolina, according to a whistleblower complaint first reported in the New York Times last year. Boeing said the Times report gave a "skewed and inaccurate picture of the program."

However, back in 2010, Boeing admitted that debris likely was the cause of a short circuit or electrical arc in a power distribution panel aboard the 787 Dreamliner that caused onboard electrical fires that led to the grounding of that jet for several months. Boeing made changes to the plane at that time because of the problem.