Leaving master switch of cruise control on does no harm

Dear Car Talk:

There are times when I discover that my cruise control is "on" but not engaged. Is driving like that doing any damage, such as lowering my gas mileage? – Michael

RAY: No. It's like having a lamp plugged in, but not turning it on. It's a safety system, Michael. Most cars have a "master" cruise-control switch. Until you set that switch to "on," you cannot engage the cruise control and use it to set the car's speed.

They want to prevent you from accidentally hitting the “set” button on cruise control while you’re driving, and then panicking when the car keeps going after you take your foot off the gas. Or, even worse, they don’t want you to hit “resume” when you’re going 45 mph in traffic, and have the car zoom back to 70 mph – where you last had the cruise control set. So they make you use two switches. Kind of like the guys in the missile silos in the Cheyenne Mountain Complex.

To use cruise control, the first switch you need to activate is the master switch. Once that’s turned on, you’ll probably see a green or orange cruise-control light on the dashboard. Only then can you use the steering-wheel controls (on most cars) to set your speed.

By the way, for years now Mercedes has been putting its cruise-control switch on a stalk that comes out the left side of the steering column. We always thought that was a bad design, because it’s pretty easy to hit it by accident when you’re using the turn-signal stalk. In fact, we’ve done that when testing Mercedes cars. And it required an immediate change of pants.

As you can imagine, Michael, a safety “on/off” switch is even more important in a car with an ergonomic issue like that. Although we wish they’d just put the controls on the steering wheel, like most other automakers now do. But to answer your question, you’re not doing any harm to the engine or to your mileage by leaving the master switch “on.” You’re just taking the first of two steps in activating cruise control.

Heat shield replacement should not be overpriced job

Dear Car Talk:

I have a rusted-out heat shield on my 2003 Honda Accord. I took my car in to get an estimate, and initially was told it would cost $319. I agreed, and set a date to bring my car in when the parts arrived. The day I took my car in, the person at the counter said that their mechanic went home sick and that they think they gave me a wrong estimate. I told them to call me when they had the correct information.

The next morning they called and said they ordered the bottom part for the heat shield, and that was in the estimate, but they still needed to order the top part of the shield, which was not in the estimate. With the extra part and an additional half hour of labor, the new cost would be $490. Before I get it fixed, do I need to replace the heat shield? And is nearly $500 the right price? That's a lot more than the original estimate. – Liz

RAY: We do recommend that you replace the heat shield, Liz. Just not with these guys.

The reason the heat shield is there is to keep your car from either catching fire or setting something else on fire if your catalytic converter overheats. Catalytic converters run very hot, even when they’re working properly. But if, for instance, one of your fuel injectors failed and started pouring gasoline into a cylinder, some of that gasoline would combust in the converter, which would then get so hot that it would literally glow.

And it certainly could be hot enough to set your car’s carpet on fire. Which might sound like a good thing, Liz, given the smell you probably have in this 15-year-old car. But the fire likely won’t stop at the carpet. A red-hot converter could also set fire to something underneath the car, like some grass or leaves you park over. Or it could ignite a piece of cardboard in your garage and burn your house down. So we do recommend you replace it. But not at the shop that wants to charge you $490. That’s way too much.

The parts, which are just pieces of sheet metal and a few bolts, should cost about $160 from a Honda parts department. And the job takes about an hour of labor. That means you should pay somewhere between $250 and $300 to have this done. So you might save half by shopping around, Liz.

I also don’t like that your shop didn’t call you to tell you their mechanic went home sick. That’s not very considerate. What if you took time off work to bring your car in? Or missed Oprah?

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