DEAR CAR TALK: When my mother drives her car, the alarm goes off and the theft deterrent light comes on. Then the car will barely run. When this happens, the power steering light, the brake light and even the tire pressure light will come on.

She then has to turn the car off, take the key out, open the door and press the unlock button on the remote to get things working again. After doing that, she can restart the car and it will be alright for a while.

We took it to get checked out and they couldn’t figure it out. She really needs your help. Thanks. – Mary

RAY: Well, that’s inconvenient, Mary. Her car thinks she’s stealing it. How sad. After all these years, it doesn’t recognize the hand that fuels it.

It sounds as if there’s a problem with the theft deterrent system, also known as the immobilizer. It’s activating while the car is being driven for some reason. And when the immobilizer is activated, it’s putting the car into what’s called “limp home mode.”

Generally, “limp home mode” is engaged by the car’s computer when there’s a danger that driving the car at normal speed will do serious damage to the engine or catalytic converter. It prevents the car from going more than a few miles an hour; enough to let you “limp” off the next exit ramp, or home if it’s nearby.

The fact that she can “reset” the car by using the remote to unlock the doors tells me it’s definitely a problem with the immobilizer. Unfortunately, on modern cars, the immobilizer is built into the car’s computer. So you can’t simply disconnect the alarm system by cutting a wire, as you could in the old days.

That means your dealer is your best bet here. The dealership is most likely to have seen this problem before. And if they haven’t, they’re most likely to know how the system works in your car, and where to start looking. And brace yourself, because it’s not likely to be a simple short in a wire, and may even require a new computer.

In the meantime, tell your mother to stop wearing a ski mask when she gets into her car. That may be confusing things. Good luck.

Got a question about cars? Email Car Talk’s Ray Magliozzi by visiting the Car Talk website, www.cartalk.com.