The best-selling compact is the subject of six National Highway Traffic Safety Administration inquiries and has been recalled nine times since it was introduced in late 1999.
NHTSA opened the latest investigations after receiving 72 complaints of engine stalling on 2000 and 2001 model cars, resulting in seven accidents. The front suspension lower control arm broke on seven cars, including 2002 models, causing six crashes.
Ford sold 574,000 Focus models in the first two years of production. A spokesman said about 600,000 cars are on the road.
"We are cooperating with the investigation," said Ford spokesman Todd Nissen. "We are getting better. We take quality quite seriously and have been taking the time to improve it."
The Focus averaged 136 problems per 100 vehicles in the most recent J.D Power and Associates Initial Quality Survey, compared to an industry average 134 problems per 100 vehicles. The survey tracks the number of complaints about a vehicle during its first 90 days of ownership.
Ford ranked fifth among automakers in quality.
Earlier Focus recalls involved inappropriate air bag deployment, air bag burn injuries, faulty rear wheel bearings, sticking throttles and engine fires.
Biologists detail four new deep-sea 'killer sponges'
Pistorius testifies he didn't consciously pull trigger when he shot girlfriend