A Ford F-150 truck goes through a quality control inspection after undergoing assembly at the Ford Dearborn Truck Plant.

Just as auto sales start to heat up, Ford is facing the daunting prospect of seeing a major drop in the supply of its best selling and most profitable vehicle, the F-150 pickup truck.

Final assembly of the F-150 is crippled by the lack of critical components after a fire knocked out production at a supplier's plant in Michigan.

Tuesday afternoon the United Automobile Workers union alerted members who build the F-150 at the Dearborn Truck Plant that production could be stopped later this week. "The company doesn't know when or for how long we will be down," wrote Burkie Morris of the UAW in a letter to members. If production is halted, Ford will likely temporarily lay off thousands of workers until it can get assembly lines running again.

The automaker has already halted production of the F-Series at its plant in Kansas City.

A spokesperson for Ford told CNBC: "We are working closely with the supplier to manage the situation."

The supplier, Meridian Lightweight Technologies, manufactures instrument panel components for the F-150.

How much could stopping production of the F-Series hurt Ford and dealers who rely on the popular pickup truck to generate huge sales?

That depends on how long the assembly lines are shut down.

Last month, Ford sold 73,104 F-Series pickups, with the overwhelming majority of those being the F-150, according to company sales reports. Meanwhile, the automaker ended April with nearly a quarter of a million trucks on the ground at dealerships, and a high percentage were F-150 models.

That inventory will keep Ford dealers stocked for several weeks, though an extended halt to production could be problematic for the automaker and painful for more than 7,600 workers.