General Motors Corp. shares were up 1.5% in premarket trading Tuesday after it said it was recalling for repair close to 6 million of its pickups and sport utility vehicles equipped with defective Takata Corp. air bag inflators.

General Motors' shares closed Monday up 4.1% after the recall announcement.

The National Highway Traffic Safety Administration said it had made the decision to order the recall Monday as the defective inflators might be life threatening.

Industry experts estimate the recall might cost General Motors about $1.2 billion - about a third of its net income this year - to replace all of the defective inflators.

Since 2016, General Motors has been trying to convince the administration not to issue a recall. The motor-vehicle manufacturer said the air bag inflator canisters remained safe on the road and in their own tests. However, owners of General Motors' vehicles accused the company of putting profits first and customer safety second.

The administration believes the defective inflators could cause further injuries. At least 27 people have already been killed by exploding inflators worldwide, including 18 people in the U.S.

The agency's investigation into the Takata air bag inflators found the volatile ammonium nitrate used to create small explosions during crashes might deteriorate over time and explode with too much force. The explosion could tear the canister holding the inflator - resulting in shrapnel.

The administration has been investigating the allegations for the past four years and it arrived at its decision this week. In a prepared statement, the administration said all available data on the inflators, including engineering analysis and age testing, supported its decision.

"Based on this information and information provided to the petition's public docket, the administration concluded that the General Motors inflators in question are at risk of the same type of explosion after long-term exposure to high heat and humidity as other recalled Takata inflators."

The administration has given General Motors 30 days to come up with an acceptable recall plan which must include a program to notify customers. General Motors said the recall was unwarranted but it would comply with the orders.

Takata's defective air bag inflators have resulted in the largest motor-vehicle recall. More than 100 million inflators have been recalled worldwide. It drove the company into bankruptcy and Takata now faces several criminal charges.