Pratt & Whitney has started carrying out tests on a design change to its F135 engine.
Deliveries of the engine have been on hold since an incident in June at Eglin Air Force Base in Florida, when an engine broke apart and caught fire as an F-35A fighter jet was preparing to take off for a training flight.
The entire F-35 fleet was grounded for several weeks. Flights have resumed, but with certain restrictions on speed.
The current testing programme is taking place at a facility in West Palm Beach, Florida, after several weeks of preliminary work.
Spokesman Matthew Bates said: “The company will conduct engine and rig testing this month to validate the suspected root cause of the engine failure and test a proposed solution.”