Ford is recalling two million of its popular F-150 pickup trucks after discovering that a seat belt component called the pretensioner has the potential to start a fire if the truck gets in an accident.

Ford says that it has received 17 reports of seatbelt-related smoke or fire in F-150s in the United States and six more in Canada. There have been no reports of injuries as a result.

What's a seatbelt pretensioner? Modern cars and trucks have sensors that can detect when a crash is underway and rapidly deploy the vehicle's safety features in a fraction of a second. In addition to deploying the airbags, a vehicle will also trigger seatbelt pretensioners that jerk the seatbelts a couple inches tighter, pulling passengers into their seats.

Some pretensioners are mechanical, but a lot of modern units use small explosive charges to rapidly tighten the belt. You can see a slow-motion video of an explosive pretensioner (not necessarily a Ford one) being activated here.

It's a remarkable bit of engineering that most people never notice—but it helps keep people safe in the event of a crash.

Unfortunately, the pretensioners in certain Ford F-150 models produced since 2014—about two million in total—are at risk of starting a fire. Federal regulators opened an investigation into the issue last month after three Ford trucks were reported "totally destroyed" in seatbelt-related fires. Thankfully, no one was seriously injured in those incidents.

"Some front seat belt pretensioners can generate excessive sparks when they deploy," Ford writes in today's press release.

The pretensioner on the Ford F-150 is inside of the B-pillar, the vertical part of the body that separates the vehicle's front and back doors. When the pretensioner activates, it exhausts gases inside of the B-pillar, and sparks can ignite these gases. "Components behind the B-pillar such as insulation and carpet may subsequently catch fire," Ford warns.

Ford is recalling trucks built between 2014 and 2018 at plants in Kansas City and Dearborn, Michigan. Affected F-150 owners can contact their local Ford dealers for repairs, which will be done at no cost to customers.

Rather than replacing the pretensioner itself, Ford's plan is to remove potentially flammable elements from the B-pillars and add flame-resistant tape, thereby preventing sparks from starting larger fires.