Q: I recently purchased a GMC 3500 with a Duramax diesel engine. Friends with diesel engine trucks are telling me of increased power and gas mileage obtained by adding aftermarket chips or tuners.

My first thought was that if they are that great, why don't the manufacturers install them as standard equipment? My second thought was, you don't get anything for free, right? So, are you harming the engine in any way by using these chips?

I eagerly await your erudite response. -- Bill

A: Well, if you want an erudite response, Bill, you're going to have to wait a lot longer. In the meantime, I'll just give you one of my usual thoughtless responses.

As VW taught us all recently, the only way you can increase power and mileage electronically is by sacrificing emissions. And that's what many of these "reprogramming devices" do.

So sure, you can get more power out of your engine. But you'll be giving the rest of us cancer, lung disease and polluted water with your NOx emissions. Not to mention it's against the law, so you'll be a criminal, too.

You're absolutely right, Bill. If the manufacturers could increase power and mileage, without breaking the emissions laws, they would have done it -- and advertised it -- before they sold you the truck.

You're right on your second point, too. You absolutely could be harming the engine. These devices can change pretty much every parameter of the engine management's system, including things like the turbo boost. If you punch up the turbo boost, don't you think there's a chance the turbo might not last as long?

And what do you think the increased force of those bigger explosions in the cylinders will do to the life of your engine?

That's why manufacturers are within their rights to void your warranty if they conclude that you've used an unauthorized aftermarket reprogramming device. They don't even have to catch you in the act. There's a lot of information stored in your car's computer these days that they can download and use to sic Robert Mueller on you.

And I think you'd be miffed if you went to your dealer after 10,000 miles with a multi-thousand-dollar engine problem and your claim got denied.

So I'd try to be satisfied with a brand new truck, Bill. That alone gives you more power and better mileage than most of us.

Q: 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

A: 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 like 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, like 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.

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Got a question about cars? Write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the Car Talk website at www.cartalk.com.