Boeing Co. (BA - Get Report) shares traded lower Monday after it suspended testing of its 777X aircraft in the latest delay to a widebody program intended to soften the impact of its grounded 737 MAX jets.

The Seattle Times first reported the 777X setback, saying a cargo door was blow outwards during the final phase of stress testing on a static plane before a scheduled certification from the U.S. Federal Aviation Administration. Boeing confirmed the suspension, but said tests would continue. Earlier this summer, Boeing said issues with the 777X's General Electric (GE - Get Report) supplied engines would push the widebody's first flights well into next year. 

Boeing shares were marked 1.9% lower at the start of trading Monday to change hands at $356.31 each, a move that would extend the stock's decline since the fatal Air Ethiopia crash of its 737 MAX on March 10 to just over 15%. 

Boeing is also reportedly working major fix to the flight software linked to its troubled 737 MAX aircraft and the escalating trade dispute between Washington and Beijing.

The software redesign involves both flight control computers and will address concerns for the system raised by the FAA in June. The fix is still expected to allow Boeing to hold to an October timetable for FAA approval, but deeper concerns with respect to the planemaker's exposure to China, as well as its deteriorating credit metrics, have pressured shares for most of the summer.

Last month, analysts at Standard & Poor's cautioned they may lower their credit rating for the world's biggest planemaker owing to the ongoing crisis in its 737 MAX program. S&P said Boeing's credit metrics are likely to deteriorate over the next few quarters, and could fall below the threshold of cash flow to debt that normally triggers a downgrade.

Boeing plans to sell bonds later this year in a six-part offering that analysts think will take priority over share repurchases at least until the 737 MAX is brought back into full service. Moody's Investors Service rated the notes at A2, while Fitch gave them a single-A grade.

Boeing posted a record second-quarter loss of $5.82 per share last week, compared to a profit of $3.33 per share over the same period last year, after booking a charge of nearly $5 billion linked to the grounding of its flagship 737 MAX aircraft, the company's most profitable plane.

Group revenues, Boeing said, were pegged at $15.8 billion, but noted that the 737 MAX charge was only an estimate of "potential concessions and other considerations to customers" owing to the 737 MAX grounding, as well as production delays, following two deadly crashes over a six month period that killed 346 people.

Boeing also said 737 production costs had increased by around $1.7 billion in the second quarter, and could likely compress margins for the flagship carrier going forward, but sees regulatory approval for the aircraft's return to service in the U.S. and elsewhere early in the fourth quarter.