A riddle: Why is a car like a toaster? A clue: Once again, we’re talking about whether it’s time to ditch my ’95 Sable wagon.

I’ve written about this car before — how I hope to keep it running until my 10-year-old son is ready to drive. How the car remains fundamentally sound despite its mileage, now at about 90,000. How keeping an older vehicle running almost always makes better financial sense than buying a new one.

But how do you know when you’re going too far? I used to judge this by my visits to the Magic Elixir aisle at Pep Boys, where you get potions to fix coughs and leaks. If you’re there more than a couple of times a year, you’re kidding yourself — the old clunker is terminal.

Later I developed a more rigorous test. The car has had it when the annual repair bills are bigger than payments on a sound replacement.

But this doesn’t get at the headache factor. If the car leaves your wife and offspring stranded beside the road, more than a couple of times, it’s got to go. And what if you have to take it to the shop over and over — even if it doesn’t cost anything?

This comes up today because the transmission rebuild I wrote about last year didn’t solve my problem. The “check engine” light has kept going on, and the car has been back at the shop four times.

The latest diagnosis is that the rebuild was fine; the problem is in an electronic black box that orders the car into fifth gear. The torque converter is not going into lock-up, whatever that means, and the car is now with an electronics specialist.

“Well,” I told my wife while driving home from delivering the wagon, “it could be just a 50-cent part.” This is the way we diehards think.

To many Americans this kind of reasoning marks you as something of a nut. But why? Suppose it is just a 50-cent part?

Even if it’s a $50 part, isn’t repair preferable to replacement? You wouldn’t replace your house just because it needed a new roof.

Or, for a better analogy, consider boats and airplanes. A friend of mine has a 46-foot aluminum ketch that was badly damaged by one of the hurricanes that hit Florida last year. He’s not replacing it, he’s fixing it.

Most people who own small airplanes have them in the shop all the time. That’s because the average small plane is 30 to 40 years old. Their owners put up with the hassles because new planes cost as much as houses, while old ones go for the price of a nice car. To replace an entire airplane over a series of electrical problems is unthinkable.

Cars are different. They are like toasters because when they break we don’t fix them, we throw them out.

Americans have been indoctrinated into this behavior by decades of car-company advertising, making us believe we’ve just got to have a cool ride. But although new is nice, it’s not necessary. I saw a wonderful documentary recently about shade-tree mechanics in Cuba who keep cars from the ’50s going.

If you don’t want to see yourself like that, think of yourself as the kind of person who could own an airplane or a yacht. Overseeing the maintenance is your job. You’re a decision maker, the kind of person who knows what questions to ask, who knows how to weigh cost and benefit. Handling these issues means you’re in control, that you’re a success.

Of course, even we corporate-executive types have our limits, and I’m not sure I’ve got the stuff for another round of big repair bills on the old Sable.

So I’m sitting by the phone, waiting for word from the shop. This could be, well … the end.

I hope not, but I know that day is inevitable.

I’ll let you know.