And Plant 2 at Oshawa, east of Toronto, is likely to shut down this afternoon, says Buzz Hargrove, president of the Canadian Auto Workers.
GM Canada spokesman Stew Low says Plant 2's fate will be decided at noon and a shutdown there could mean layoffs for another 3,000.
The Oshawa truck plant, with 4,000 employees, is "good until Thursday," Hargrove said in an interview.
He had warned yesterday that tens of thousands of workers at GM Canada and in the Canadian auto parts industry would be immediately hit by the impact of the strike against GM in the United States.
Workers on the overnight shift at Plant 1 were sent home at about 3 a.m., about four hours before the scheduled end of their shift. The plant, which produces the Impala and Monte Carlo models, ran out of engines made in the Untied States.
Hargrove said the second auto plant, which produces the Grand Prix and the Allure, is likely to shut down at the end of the day shift at 4 p.m.
He said the truck plant has enough parts to last about three days, so it will close late Thursday or early Friday unless the strike in the U.S. is settled before then.
The GM Canada transmission plant in Windsor, Ont., closed yesterday within an hour of the U.S. strike beginning and Hargrove says the company's engine and transmission plant in St. Catharines, Ont., will close by the end of the week.
The Windsor plant has 1,300 workers and the St. Catharines plant employs 3,000.
In a move that surprised many industry watchers, about 73,000 UAW members walked off the job at GM's assembly and parts operations across the U.S. yesterday morning after a 10-day extension of a strike deadline for a new contract. It is the first national strike at GM in the U.S. in 37 years.
"I'm shocked," said Gerald Fedchun, president of the Automotive Parts Manufacturers' Association. "It's like two guys who are chained together deciding they're going to jump in the water."

The strike is creating anxiety for GM workers, who already face the loss of more than a 1,000 jobs at the truck plant by the end of the year.
"I've got a wife and kids. Do I worry? Sure, but there's nothing I can do about it," said Greg Cutrara as he came off work at the truck plant. "You've got to hope for the best."
Other workers expressed optimism the U.S. strike would be settled before there's too much pain.
"It all depends on how long it lasts," said Peter Chapman, who works for an industrial cleaning firm at GM's Oshawa operations.
Idle GM workers are eligible for federal employment insurance and company supplementary unemployment benefits that total about 65 per cent of their regular gross pay. That pay would start in the second week of any layoff.

Industry watchers said Hargrove's estimates regarding the impact on Canadian parts plants and workers may be slightly high, but they agreed the ripple effect of the U.S. labour action will be significant and swift.
GM Corp., the world's second biggest vehicle maker, uses close to 50 per cent of Canada's independent auto-parts output.
GM assembly plants here need components that the auto giant produces in the U.S. while domestic parts makers supply the company's vehicle operations south of the border. The assembly plants hold little inventory so a strike would quickly eliminate any demand.
"The impact on Canada is immediate," said analyst Dennis DesRosiers. "This could very quickly get north of 50,000 direct jobs."
Fedchun said the strike will force big GM suppliers such as Magna International Inc., Lear Corp., Woodbridge Group, ABC Group and Martinrea International Inc., to cut operations drastically.

Furthermore, it will affect other smaller companies that provide parts and services to the upper tier suppliers, he said.
"By the end of the week, the whole parts industry could be shut down to a large degree," Fedchun said.
Magna spokesperson Tracy Fuerst acknowledged that the Aurora-based industrial powerhouse may need to "suspend the supply of parts to GM" if the strike is long.
Hargrove said although he is not at the bargaining table in Detroit, the chances of a quick settlement do not look good because the two sides have talked for 21 consecutive days.
GM had 67 days' worth of vehicles at U.S. dealerships on Sept. 1 in comparison to 57 days for the industry, according to the trade journal Automotive News. However inventory of some popular models is lower, which could result in supply shortages if the strike continues for more than a month.
The GM strike will mean a production loss of about 16,000 vehicles daily in the U.S. and Canada.

GM, which lost $12.4 billion (U.S.) in 2005 and 2006, is trying to reduce a labour cost disadvantage of $25 an hour with foreign rivals.
The UAW has shown signs of agreeing to a new retiree health-care fund to reduce GM's cost burden, but the union wants job security in return and is resisting other concession demands.
In 1998, an eight-week strike at two parts plants in Flint, Mich., cost GM more than $3 billion because it created component shortages that idled most of the firm's operations.

With files from Tony Van Alphen and Chris Sorensen.