New muffler and exhaust pipe, $400. New tires, $700. Third trip to find out why the “check engine” light is still on, $150. The mechanic is thinking the engine may need a head gasket. Estimate: $1,200. Trade-in value on old clunker? Priceless, in the sense that it has no value at all, not in the sense that it is worth a lot.

How do you decide when to trade in a car? Maybe I’ve made all the repairs and it will run for another 98,000 miles. Or maybe this is just the beginning, and it will need the new head gasket and then a new transmission and then a new radiator, and then the generator will go out before the thing stops running altogether.

I’ll never be able to sell it with that check-engine light on. The good news is that sometimes it goes off. Maybe I’ll get lucky and the light will be off when I try to trade in the car. Would that be wrong? After all, it has a new muffler and new tires; someone would be getting a practically new car.

Why do I have to go through this every six or seven years?