Q: I was just checking out my daughter’s new car and was told it has run-flat tires. Can you explain how these work?

A: Run-flat tires are designed so that, in the event of a puncture or the tires losing air, they can be driven up to 50 miles (or farther in some cases) at speeds below 50 mph. This allows the automaker to eliminate the spare tire and car jack, saving weight and space — and money. Run-flats can also bring peace of mind to folks who aren’t comfortable with the idea of changing a tire along the road. Run-flats also offer greater vehicle stability should a sudden loss of pressure occur.

Stiffer and stronger sidewalls are how it’s done. This does come at a price, as the heavier tire weight and stiffness somewhat affects handling and ride smoothness. There’s also higher initial cost and, with certain brands, the tires are nonrepairable. Shorter tread life seems the norm also, possibly due to a softer rubber used for the tread. Switching back to conventional tires isn’t advised on some vehicle makes as the suspension systems are tuned for the stiffer sidewalls.

In a sports car or compact vehicle where a spare tire can’t be done run-flats make sense to me. Given a choice, I’d prefer conventional tires or perhaps self-sealing tires. These feature conventional construction and include a sealing layer inside that can self-repair most tread-area punctures. Continental and Pirelli both market self-sealing tires.

Q: My engine light goes on and off about once a week. I think I remember you saying this could only be fixed if the vehicle’s acting up when I bring it in. Is this true?

A: Let’s clarify how intermittent fault diagnosis might play out. Your vehicle’s onboard diagnostic system will keep a diagnostic trouble code and engine data freeze-frame in memory for at least 40 drive cycles after a fault goes away. If you were to bring the vehicle in for repairs during this period, a technician could retrieve this information and determine the general area of the problem. A typical trouble code might indicate a fault in a circuit or component, an issue with a certain engine cylinder, incorrect air-fuel ratio, unacceptable fuel system vapor purging or loss, or perhaps an emission control device that isn’t performing up to par.

The data freeze frame provides exact driving conditions at the time the trouble code was set. Driving the vehicle under the same conditions is the best way for a technician to duplicate the fault, and after repairs, to make sure it is fixed.

Unless the fault can be duplicated, it’s difficult, if not impossible, to accurately determine the cause. Throwing a part at the problem because of a previously stored code without a confirmed diagnosis is asking for a costly come-back. Do your best to establish a pattern or likely conditions that can help the technician duplicate the fault.