By Ben Foldy and Mike Colias
Ford Motor Co. stopped assembly lines at key factories in Chicago and Michigan on Wednesday, the latest sign of the risks to companies and their employees as they attempt to resume work during the Covid-19 outbreak.
Ford sent Chicago Assembly Plant workers home for a second day in a row late Wednesday morning, people familiar with the matter said. The work stoppage came after a Lear Corp. factory in nearby Hammond, Ind., that makes seats for the Ford factory idled its assembly lines, the people said.
Also Wednesday, Ford temporarily closed a pickup-truck plant in Dearborn, Mich., near its headquarters, after a worker at the plant tested positive for Covid-19, a company spokeswoman said. Work is expected to resume Wednesday night after part of the factory is disinfected, she said.
The Dearborn plant is one of two factories that make the F-150 pickup, Ford's biggest moneymaker. The Chicago plant makes the Ford Explorer sport-utility vehicle, one of the company's most popular models, along with the Lincoln Aviator SUV.
On Tuesday, Ford was forced to close the Chicago factory twice for several hours after confirming two positive cases of Covid-19 at a nearby parts-assembly facility, the company spokeswoman said. The main assembly plant was back in operation by Wednesday morning after both facilities were disinfected overnight.
The spokeswoman said the company believes the workers already were infected when they returned to the job this week.
"Due to incubation time, we know these employees did not contract Covid-19 while at work," the spokeswoman said. "Our protocols are in place to help stop the spread of the virus."
Texts sent to employees at the Lear plant and viewed by The Wall Street Journal said that an employee on the first shift tested positive for Covid-19. The facility has since been disinfected and the night shift is to report as scheduled Wednesday, the texts said.
A spokesman for Lear Corp. didn't immediately respond to requests for comment.
The closures come after Detroit's auto makers on Monday began restarting their U.S. factories, which were idled around March 20 as the coronavirus pandemic took hold. The companies have spent several weeks preparing measures to ensure a safe work environment. They include temperature checks, plastic barriers between work stations and even electronic bracelets that beep if an employee violates social-distancing rules.
Executives and analysts have warned that the industry's restart will be slow and complex, including the risk of infections popping up at auto makers' factories and at those of suppliers.
The car companies have resumed operations with far fewer workers making vehicles in extremely small volumes as they gradually resume operations. Executives have said production will start slowly as workers familiarize themselves with safety measures, parts trickle in from suppliers and machinery is tested after the long idle period.
The two Ford employees who tested positive had passed an initial temperature check when they arrived for their shifts on Tuesday, the company spokeswoman said. It was unclear how they later tested positive during their work shifts, she said. Employees who had close contact with the workers were asked to self-quarantine for two weeks.
Ford experienced a troubled rollout last year of a redesigned Explorer and the Aviator, a new SUV model, after a major overhaul of the Chicago factory. Executives cited the problematic launch as a major reason that Ford fell short of its profit target in 2019, but have said the factory now is operating smoothly.
--Austen Hufford contributed to this article.
Write to Ben Foldy at Ben.Foldy@wsj.com and Mike Colias at Mike.Colias@wsj.com