After the earthquake hit, car companies began the long process of figuring out which parts are in danger of running out. That means figuring out where every piece in every part comes from.
"Everyone is putting on the brakes a little bit and taking a look to see where they are affected," says Paul Newton, an analyst with IHS Automotive.
Companies will shut down plants as soon as some parts start running out, which could start happening in the next four to six weeks, he says. "You will see it happen almost daily."
IHS Automotive predicts that one-third of daily global automotive production will be cut because of supply chain disruptions. That means about 5 million vehicles worldwide won't be built, out of the 72 million vehicles planned for production in 2011.
To get a feel for the supply chain, consider a car radio. It's made up of hundreds of pieces from all over the world. The display may come from a supplier in Japan, while the wiring and circuitry originate in Korea. The plastic knobs could come from a company in China, and the metal structure that holds it all together is shipped from India.
All those parts come together at different times: The wiring and electronic components are installed into the metal frame. Then that piece is shipped to another supplier, who snaps on the plastic face and knobs. The radio could pass through three or four suppliers before being put on a ship, where it will spend weeks at sea heading to its final destination: The assembly plant.
"This isn't just as straightforward as assembling the iPad 2," says Brian Johnson, an autos analyst with Barclays Capital.
An example of Japan's importance in auto parts: its suppliers make many of the electronic components that control music systems and the sensors that monitor fuel levels and airbags.
Although most Japanese auto parts makers are not located in the areas that were inundated by the tsunami, between quake damage, electricity outages and water cutoffs, many factories in the region have remained paralyzed ever since.
Suppliers could be up and running again in April, but it could take until May or June for the entire supply base to be back.
Some car manufacturers, meanwhile, are considering shifting operations to deal with the crisis. Nissan, for example, is thinking of moving some of its engine production to Tennessee from Japan.
But those shifts won't be easy. First, lean inventories make it hard for automakers to suddenly change sources of supply. And plants that build car electronics, for example, have stringent safety requirements and exacting high-tech specifications that limit a company's flexibility, says Christopher Richter, an industry analyst at CLSA Asia Capital Markets. A supplier for the computer chip that triggers an air bag, for example, can't be switched quickly.
But car executives can keep this from becoming a total disaster: They can allocate scarce parts to their more popular or profitable vehicles, keeping those assembly lines running while slowing down the less profitable ones.
That's what many people believe GM did when it decided to close the Shreveport plant, because dealers have ample inventory of both trucks made there, more than two months' worth.
Newton says car companies will do their best to keep producing the cars people want.
"It's quite a lot to prioritize, but they'll do it," he says.