The US government’s auto safety regulator has ended a 2½-year investigation into Ford engine failures after the company replaced engines or extended warranties on some vehicles.

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

The documents say the probe, which opened in May 2022, has grown to cover 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 investigating catastrophic engine failures caused by ruptured intake valves, cylinder drop and piston knocking.

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

A review of defect report data found the faulty valves were manufactured from May to October 2021, the agency said. Ford claimed 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.

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 most vehicles with defective valves “have already experienced a malfunction,” the agency said.

Earlier this year, Ford recalled about 91,000 vehicles with valves manufactured during the suspect period. They will be tested and given a new engine if necessary. The company also extended the warranty on valve vehicles manufactured during the period to 10 years or 150,000 miles, the filings said.

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

— The Associated Press