•Q. I met my fiance in a car accident three years ago. I bumped into his car, and the rest is history. However, as luck would have it, the only time we argue is when it comes to driving and car care. He drives a stick-shift Mazda Miata, and I drive an automatic 2010 Nissan Altima. So there are a lot of features in my car that he had to get used to (e.g., keyless ignition). The one thing that always gets me is that he often turns off the engine without shifting back into park. I tell him that it damages the car, and he says no it doesn’t. I find that hard to believe, because the car won’t start if the gear lever is not in park. Can you knock some sense into my man? I love him to death, but not his car/driving philosophy. Thank you.•

RAY: Well, the act of turning off the engine while the car is still in drive doesn’t hurt anything, other than the chances that you’re going to go with him to the altar. Neither the transmission nor the engine care.

TOM: But the reason he should break himself of this habit is because it’s easy to then forget to actually put it in park.

RAY: Maybe you’ve done this yourself at some point? You pull up somewhere, you’re a little distracted, so you turn off the ignition and take your foot off the brake. Then in your peripheral vision you notice that the car next to you is moving. And suddenly it occurs to you: It’s not the other car, it’s me!

TOM: Of course, it would be worse if you had gotten out of the car before it started to roll away. Luckily, the keys won’t come out of the ignition unless the car is in park. So that gives you one more signal that something’s wrong (why won’t my key come out? Oh, right. Because I didn’t put the car in park, and my car is rolling into a UPS truck!).

RAY: But in a car like yours, with keyless ignition (where you just need to have the key in your pocket, and the car recognizes it electronically when you get in the car), it’s one step easier to leave the car in drive, open the door, get out and walk away.

TOM: Now, that presupposes that you’d somehow miss the warning chime that would sound when you opened the door with the car in gear. But remember, you’ve already tried to turn off the car without putting it in park, so we know you’re distracted.

RAY: So, while it’s not mechanically harmful, it could lead to a stupid mistake. So remind him that you’re getting married soon. And tell him that means that if he does ever leave the car in drive and lets your car roll into a fetid, alligator-infested swamp, there are a lot of years ahead for him to never live it down. Good luck.

•Q. I have a Dodge Durango, 2000, four-wheel-drive. I have three faulty oxygen sensors (as detected by a scan); one of them may have oil-soaked wires. I’d like to replace them but have been told by a mechanic friend that I really ought to get actual Dodge parts from the dealer, because “Dodges don’t like aftermarket parts from O’Reilly’s, Advanced, etc., auto parts stores.” An auto rebuilder friend of mine says that’s a bunch of nonsense, and that the aftermarket parts from O’Reilly’s, et al. are just fine. There’s a significant price difference, and I’m reluctant to ask Dodge about this, as I know what the answer will be. So, what do you think? Original factory oxygen sensors, or aftermarket, more affordable ones?•

RAY: There’s no single easy answer to this question about factory parts versus aftermarket parts.

TOM: Lots of aftermarket parts are just fine. In fact, some are exactly the same as the original parts, and are made by the same supplier. We’ve run into cases where the original supplier is, for instance, DENSO, and the same DENSO part is available online cheaper than at the dealer’s parts department.

RAY: But it’s also possible to get cheap junk. We’ve had situations where aftermarket parts didn’t perform the way they were supposed to.

TOM: In this case, I think it’s fine to take a chance on aftermarket parts. Why? Because the labor is so minimal. An oxygen sensor is basically a plug-in part. It takes a minute to install (although wrestling out an old, rusted one might take a while!).

RAY: So if, by chance, the part you get doesn’t do its job, you can unplug it, take it back for a refund and buy something else. Because aftermarket parts usually are guaranteed.

TOM: It’s just that the labor is not. So if you were installing a clutch that took a weekend of labor, you wouldn’t want to mess around with aftermarket parts unless you were really certain of them, because that’s not a job you want to do twice, even if the second set of parts doesn’t cost you anything.

RAY: But in the case of things that are relatively easy to install and remove, especially on an older vehicle, I wouldn’t hesitate to try a well-regarded aftermarket part.

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