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Washington — The National Highway Traffic Safety Administration said Friday it is upgrading its investigation into 20,275 Ford Explorer Police Interceptor SUVs over sudden braking failures.

NHTSA first opened a preliminary investigation in April to investigate reports of low-mileage failures of front brake jounce hoses in 2015 Ford Explorer Police Interceptor SUVs operated as training vehicles by the City of Sacramento, California. NHTSA's decision to upgrade the probe to an engineering analysis is a step required before the agency can issue a formal demand for a recall.

The Sacramento police fleet has reported seven front hose failures in five different Explorer Police Interceptor SUVs used in its Emergency Vehicle Operation Course training. Most of the failures took place within the first few miles of service on a closed course used for the program, which includes evasive accident avoidance and pursuit maneuver training.

The hose assemblies either leaked or pulled completely free from the caliper-end attachment where the hose is crimped to the steel end fitting. Most of the failures resulted in a sudden loss of braking performance that caused the vehicle to run off the intended course.

None of the failures resulted in any crashes, injuries or property damage. The 2015 hose assemblies are similar in design to hoses used 2013-14 Explorer Police Interceptor and civilian vehicles. The five vehicles used by Sacramento EVOC have all been repaired using a new 2016 front hose assembly that have been changed to incorporate a short steel tube attached to the caliper end banjo block.

No failures have been experienced by the Sacramento fleet to date in the other 37 2014-15 Explorer Police Interceptor vehicles used by the fleet in regular law enforcement duty cycles unrelated to the EVOC training.

Ford told NHTSA that two other similar failures have been reported by other fleets, including one involving a Sacramento Regional Transit vehicle that occurred while being driven on the street. Ford attributes the Sacramento fleet hose failures to excessive temperatures produced during "hot-soak" portions of the training duty cycle — when vehicles are stationary after hard-braking exercises, with no cooling air flow through the brake rotor vents and across the other brake components.

During testing conducted to replicate the Sacramento course, Ford measured temperatures at the subject crimp fittings that exceeded the design limits of the brake hose material after several successive training intervals and hot soaks.

Ford has given the Sacramento fleet modified replacement hose assemblies for all of its 2014-15 Explorer Police Interceptor vehicles.

"The replacement hose assemblies were developed for use in 2016 Explorer vehicles and were modified slightly to accommodate the addition of a ride height sensor in those vehicles," NHTSA said. "Ford believes that the excessive temperatures experienced at the crimp fitting in the subject vehicles are unique to the EVOC duty cycle, have not been observed in the standard on road severe duty cycle testing performed by Ford and police fleets who routinely conduct such testing and are not likely to occur in service usage for on-road Explorer Police Interceptor vehicles."

“We will cooperate with NHTSA on this investigation, as we always do,” Ford spokeswoman Kelli Felker said.

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