Pratt & Whitney plans this month to start testing a potential fix to the engine problem that caused a Lockheed Martin Corp. F-35 fighter jet to catch fire in June.

Investigators have yet to pin down the cause of the accident, which led to the temporary grounding of the entire F-35 fleet, with flying restrictions still in place that threaten to trigger another delay in the plane's planned entry into combat service next year.

Pratt...