Hyundai Motor Company has admitted that its most popular vehicle sold in the United States has a serious airbag flaw that it doesn’t know how to fix, ABC News reported.

The problem, in the company’s 2004 Elantra, reportedly is a sensor system intended to prevent the airbag from deploying while a child sits in the front seat, but it’s also preventing the airbag from activating for adults who weigh less than roughly 150 pounds and don’t position themselves in the centre of the chair.

“I’m worried. I’m seriously concerned that this isn’t a safe car,” Charlotte Kramer from Fort Lauderdale, who purchased her 2004 Elantra late last year, told the TV news service. “It’s a risk every time someone sits in the car as to whether their passenger side airbag is going to be activated or not.”

ABC News said Kramer became concerned when she saw a warning light appear on her dashboard that read: “Passenger Airbag Off.” The light stayed on when an adult weighing less than 150 pounds sat in the passenger seat off-centre — meaning that the airbag would not deploy for the passenger in the event of an accident.

Kramer reportedly sought help at the dealership where she purchased the car but she was told there’s no way to fix the problem, which could mean the airbag would not have deployed in an accident.

Hyundai spokesman Mike Anson told ABC News the only thing passengers not heavy enough to activate the airbag can do now is sit somewhere else.

“Either move to the back seat or move to another Hyundai vehicle,” he reportedly said.

In a technical service bulletin obtained by ABC News, Hyundai advises service technicians to tell customers to try the following in an attempt to activate the airbag: “Turn the vehicle off, place the seat back in the full upright position, sit upright in the seat, centred on the seat cushion, with legs comfortably extended. Restart the vehicle and have the person remain in this position for about 30 seconds.”

Agreeing that the instructions may be a bit too much for many customers to remember, Anson reportedly suggested that new customers take his company’s cars for a long test drive before making a purchase.

“Before you buy the vehicle, put the people in the front passenger who will be in there to make sure they’re compatible,” he told ABC News. “That way you won’t have a problem.”

The report said the National Highway Traffic Safety Administration, which now requires some new cars to use a sensor system for passenger-side airbags, is investigating Hyundai’s problem and response – it reportedly said it needed to investigate more before determining whether there’s a need for a recall of the vehicles.

Hyundai told ABC News it has sold 50,000 2004 Elantras.