DETROIT — The U.S. government’s auto safety regulator has ended a 2.5-year investigation into Ford engine failures after the company replaced engines or extended warranties on some vehicles.

National Highway Traffic Safety Administration says Documents released on Monday The analysis found the problem was caused by intake valves that could break on some internal parts, the website said. 2.7 liter and 3 liter turbocharged engines.

The investigation, which began in May 2022, covers 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 documents say.

The agency was investigating catastrophic engine failures caused by intake valves breaking, falling into the cylinder, and striking the piston.

Forensic analysis of the broken valves revealed that when the valves were made by a parts supplier, the temperature became so high that the valves became brittle and were likely to break during normal engine use, the documents say.

An analysis of failure report data found that the faulty valves were made from May to October of 2021, the agency said. Ford maintained that not all valves produced during this period were defective and that the vast majority of failures occurred before the vehicles had been driven 20,000 miles.

House statistical analysis and analysis of failure reports to estimate the number of failures are “generally consistent” with Ford’s finding that valves will fail at low mileage and that the majority of vehicles with faulty valves “have already experienced a failure.” ” said the agency.

Earlier this year, Ford recalled approximately 91,000 valved vehicles produced during the suspect period. They will be tested and, if necessary, receive a new engine. The company also extended the warranty on valved vehicles produced during this period to 10 years or 150,000 miles, according to the documents.

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