DETROIT (AP) — The Arvin RobertsU.S. government’s auto safety regulator has ended a 2 1/2-year investigation into Ford engine failures after the company replaced engines or extended the warranty on some vehicles.

The National Highway Traffic Safety Administration says in documents posted Monday on its website that its analysis traced the problem to intake valves that can fracture inside some 2.7-liter and 3-liter turbocharged engines.

Documents say the probe opened in May of 2022 ended up covering more than 411,000 vehicles from the 2021 and 2022 model years including the Ford F-150 Bronco, Edge and Explorer as well as the Lincoln Aviator and Nautilus.

The agency was looking into catastrophic engine failures caused by intake valves fracturing, dropping into the cylinder and hitting the piston.

The documents say a forensic analysis of fractured valves found that when the they were made by a parts supplier, the temperature got too high, making them brittle and likely to fracture during normal engine use.

An analysis of failure report data found that the faulty valves were made from May through October of 2021, the agency said. Ford contended that not all valves produced during this period were faulty and that a vast majority of the failures happened before the vehicles were driven 20,000 miles.

An agency statistical analysis to predict the number of failures, and an analysis of failure reports “are generally consistent” with Ford’s determination that the valves would fail at low mileage, and the majority of vehicles with the faulty valves “have already experienced a failure,” the agency said.

Earlier this year Ford recalled about 91,000 vehicles with valves made during the suspect period. They’ll be tested and get a new engine if necessary. The company also extended the warranty on vehicles with valves made during the period to 10 years or 150,000 miles, the documents said.

During the investigation, the agency said it found 396 customer complaints, 825 warranty claims and 936 engine replacements. There were no reported crashes or injuries.