Ford Motor Co.’s recall of millions of late-model trucks and SUVs calls into question the quality of its popular line of work vehicles. The Sept. 7 announcement also reveals how sensitive components, poorly designed and maintained, can cascade into serious problems.

The 4.5 million vehicles span nearly a decade, starting in 1994. They include Ford F150 and F250 pickup trucks. They may catch fire due to a faulty cruise control switch, which is located near the master cylinder. Ford says that brake fluid may leak through the switch, corroding it and causing a fire. The huge recall expands on a smaller re-call in January and now is the fourth largest in U.S. history.

Last year, reports of under-the-hood fires led Ford and the National Highway Traffic Safety Administration to launch separate probes. The investigations were complex "because the root cause turned out to be a system interaction rather than a single component," says Ray Nevi, assistant director of Ford’s safety office. For the fix, Ford will install a fused wiring harness, which acts like a circuit breaker.

Coincidentally, General Motors Corp. has recalled 804,000 pickup trucks and SUVs, whose antilock brakes may suddenly deploy. Toyota recalled 978,000 pickups and SUVs, which may develop fatigue cracks in steering relay rods.