Dear Car Talk:

What’s the purpose of knee air bags? I was in an accident that resulted in the knee air bag doing major damage to my left leg. The right leg was on the brake and sustained bruising but not terrible problems. — Sondra

When they were introduced, the feeling about air bags was “the more, the merrier.” After all, right behind your dashboard is steel, and if we can provide a cushion between your knee and steel, why not, right?

The problem is that our federal safety regulators have a mandate to protect two different populations: the belted and unbelted.

So, when a car gets “crash tested,” they have to test it with both a dummy that’s wearing its seat belt, and a complete dummy that’s not wearing its seat belt. And in order to pass both of those tests, automotive engineers have to make compromises.

In the case of knee air bags, engineers figured out that an air bag at the knees could help keep an unbelted dummy in a more upright position during a crash, so he wouldn’t slide under the steering wheel and get crushed to death.

Unfortunately, that probably required a larger and more powerful knee bag than was necessary just to protect the lower legs of the belted majority of drivers. A 2019 study by the Insurance Institute for Highway Safety demonstrates that.

IIHS studied real-world crash data from 14 states. And they found that for drivers and passengers wearing their seat belts, knee air bags barely helped prevent injuries, and in some types of accidents, they increased the risk of lower-leg injuries.

So what to do? It’s a public policy question that’s beyond the purview of this crash test dummy. But if it were up to me, I’d focus on the people who wear their seat belts and issue everybody else a football helmet and wish them the best of luck.

I’m really sorry you were injured, Sondra. I hope you heal up quickly and completely.

Dear Car Talk:

What causes my wife’s lowmileage 2013 Honda Civic SI’s air bag warning light to come on occasionally? For the past several months, the light has been coming on after driving for a short time or

sometimes when the vehicle is first started.

The local dealership estimated the repairs, which included pulling the steering wheel, would be about $500. I discovered that yanking on the shoulder seat belt several times would cause the warning light to disappear for several days, but that the light would eventually return.

Is there a bad connection in the shoulder seat belt system? Is there a quick fix for this problem? — Reid

I think you should ask the dealer for a little more information before forking over the $500. He wants to remove the steering wheel, which suggests he thinks the problem is something related to the air bag itself, the clock spring in the steering column or a connection in that vicinity.

If yanking on the shoulder belt while you’re wearing it reliably makes the light go off, then the problem may not be in the steering column. It may be the seat belt latch. The latch, near the driver’s right hip, where you insert the seat belt clip, contains a tiny switch that lets the computer know that your seat belt is being worn. If that switch is dirty or out of adjustment, that would cause your air bag light to come on.

The problem could also be at the other end of the seat belt, where it spools up. There’s a pretensioner there that cinches the belt tighter in an accident, to keep you in a better position to avoid injury. If there’s a problem with the pretensioner, your air bag light will come on, too.

So start by asking the dealer for a more specific diagnosis. Ask him if he scanned the car, and if he did, what did he learn? Ask what, exactly, he thinks is causing the problem, and what the fix entails. If you’re not convinced, ask another Hondafriendly shop to scan the car for you, and see what information that turns up. It may tell you exactly what part is faulty.

If it turns out to be a fault with the switch inside the latch — that’s something that any good mechanic can try to clean for you. But if it’s more complicated than that, I’d put on your Kevlar pants and go to the dealer. First of all, Honda offers a lifetime warranty on their seat belts. So if it’s something like the pretensioner, your repair may be free.

Second, air bags are pretty important. They can mean the difference between life and death. So when you’re dealing with key safety technology, it makes sense to go to a place that has the experience and the tools to do it right. And the liability insurance to make a big payout to your heirs if they muck it up.

Got a question about cars? Write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the Car Talk website at www.cartalk.com.