Less than a month after General Motors announced that 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 GM 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 completing the repairs. The repairs will not begin, GM said, until early April

On Friday, GM started sending out the first recall letters to registered owners, telling them that a fix is 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.”

That’s 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. GM said it had linked the defect to 31 crashes and 13 deaths since it was first alerted to the problem in 2004.

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

For the older cars being recalled by GM, simply getting the part made is a challenge for the automaker.

The recall covers six models: 2005-07 Chevrolet Cobalts; the 2007 Pontiac G5; 2003-07 Saturn Ions; 2006-07 Chevrolet HHRs; 2006-07 Pontiac Solstices; and the 2007 Saturn Sky.

Alan Adler, a GM spokesman, said that the supplier, Delphi, needed to prepare the machines that would make the part before it could be mass-produced.

In some recalls, parts suppliers have already sold off those machines, making the recall process even more time-consuming. 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, GM 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 over the past seven years, its investigations have resulted in more than 900 recalls, covering more than 50 million vehicles.

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.

For models such as the Cobalt, GM 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.

“These are second- and third-owner cars now, because of their age,” 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.”

Adler said General Motors would soon establish a website where owners could get more information and that GM has monitored Facebook discussions of the Cobalt as well. And, he added, GM will mail follow-up postcards.

In fact, the government specifies that manufacturers send the recall notices by mail, though the NHTSA will also let consumers sign up to be notified on Facebook and Twitter.

Oddly, the recall covers some parts that GM 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, Adler said, but Delphi did not change the part number. It did change the part number for the following model year, but the only way to catch all the bad parts in the 2006s is to replace every one of them, he said.

The automaker would not say how much the recall 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.

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

Getting the parts, he said, is an issue in every recall. He is 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 Fox, who is vice chairman of the National Automobile Dealers Association.

GM, 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.

Fox said he would have staff members telephoning owners identified by GM, though this was not a perfect system.

“How do you notify that guy,” he added, “if you don’t know who he is?”