Less than a month after General Motors announced it would recall 1.6 million cars because of a defective ignition switch, the automaker now faces an arduous task: fixing the cars.

The process, particularly for older vehicles like the ones G.M. is recalling, is time-consuming and requires many steps, from designing the new parts, testing them to make sure they solve the problem, finding and informing owners, and actually completing the repairs. It will not be until early April, G.M. said, that the repairs will begin.

On Friday, G.M. started sending out the first recall letters to registered owners, telling them that a fix was coming.

Under the heading “Important Safety Recall,” the letter contains a stern, if unusual, warning: “Remove all items from your key ring, leaving only the vehicle key.”

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That is because if the defective ignition switch is jostled, or even if the key chain is too heavy, it can turn off the engine and the car’s electrical system, disabling the air bags. G.M. said it had linked the defect to 31 crashes and 13 deaths since it was first alerted to the problem in 2004.

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The 2005 Chevy Cobalt, shown at a Detroit car show, is among 1.6 million cars recalled by G.M. Credit Gregory Shamus/Reuters

The letter, which does not advise drivers to stop using their cars, also tells owners that the replacement parts “are not currently available.”

For the older cars being recalled by G.M., simply getting the parts made is a challenge for the automaker. The recall covers six models: 2005-7 Chevrolet Cobalts; the 2007 Pontiac G5; 2003-7 Saturn Ions; 2006-7 Chevrolet HHRs; 2006-7 Pontiac Solstices; and the 2007 Saturn Sky.

Alan Adler, a G.M. spokesman, said the supplier, Delphi, needed to prepare the machines that would make the part before mass production could begin.

In some recalls, parts suppliers have already sold off those machines, making it even more time-consuming. Mr. Adler said he did not know if that was the case for the ignition switches.

Once the part is made, a second letter will be sent to registered owners telling them to schedule an appointment at a dealership. That letter, G.M. said, will go out later this month. The parts are not expected to arrive at dealerships any sooner than early April.

The recall of the 1.6 million vehicles, including nearly 1.4 million in the United States, is large, but it is one of many. The National Highway Traffic Safety Administration said that in the last seven years, its investigations have resulted in more than 900 recalls, covering more than 50 million vehicles.

The recalls are highly varied, and some are quite small. This month, for example, the agency announced the recall of 30 horse trailers, which could potentially lock a person inside.

Most recalls, including G.M.’s current one, are announced by automakers; in some cases, regulators order one. Toyota, for example, was required to recall millions of vehicles in 2009 and 2010 after consumers complained of unintended acceleration.

And not all cars subject to a recall actually get repaired.

“On brand-new models, you get 100 percent,” said Greg Martin, the chief spokesman for General Motors. On older cars, sometimes the carmaker no longer has the address of the current owner, or the car may no longer be on the road.

“Something like this, if you do 70 or 80 percent, that’s really great,” Mr. Martin said.

In January, for example, G.M. recalled 370,000 new pickup trucks for a software error that could cause fires, and 80 percent were fixed in the first five weeks, according to the company.

For models like the Cobalt, G.M. will use an automotive consulting firm, R. L. Polk, which performs computer searches of state motor vehicle registrations to find current owners through vehicle identification numbers. But they are hard to find.

“These are second- and third-owner cars now, because of their age,” Mr. Adler said. “It may have been through a private sale, or a used-car lot; you could have bought a Cobalt at a Ford dealership where somebody traded it in.”

 

And some of those who are reached may not respond, he predicted. “Not everybody reads their snail mail anymore,” he said.

Mr. Adler said General Motors would soon establish a website where owners could get more information, adding that it had monitored Facebook discussions of the Cobalt as well. And, he said, G.M. will mail follow-up postcards.

In fact, the government specifies that manufacturers send the recall notices by mail, although the N.H.T.S.A. will also let consumers sign up to be notified on Facebook and Twitter.

Oddly, the recall covers some parts that G.M. knows are good. Around November 2006, in the 2007 model year, Delphi changed the part so that it was no longer vulnerable to turning the car off, Mr. Adler said, but Delphi did not change the part number. It did change the part number for the next model year, but the only way to catch all the bad parts in the 2007 models is to replace every one of them, he said.

While the recall is an expense for G.M., the automaker would not say how much it would cost. And though the repairs will be done free, it is at least an inconvenience for owners, who need to make an appointment and take their car in for repair.

William C. Fox, who owns Honda, Toyota, Subaru, Chevrolet and Chrysler dealerships around Syracuse, N.Y., said that a few of the people who brought in cars would be “orphan owners,” because the cars are Pontiacs, which are no longer made.

Getting the parts, he said, is an issue in every recall. He is currently conducting a recall of Toyotas with defective heated seats. “The manufacturer is making three or four a day for our area, and we have 20 people waiting,” said Mr. Fox, who is vice chairman of the National Automobile Dealers Association.

G.M., Mr. Fox said, was better than some other carmakers; he predicted it would have its parts suppliers working overtime.

And the recall could even be a plus for dealers, who will see customers they normally do not see. “This is good business, as long as we can get the parts,” he said.

Mr. Fox said he would have staff members telephoning owners identified by G.M., although this was not a perfect system. “How do you notify that guy,” he added, “ if you don’t know who he is?”