Dear Car Talk:

My 2007 Toyota Sienna has a Tire Pressure Monitoring System. It recently alerted me to a leaking tire, and I found the nail in the tire and took it to be fixed. The mechanic removed the tire, because he claimed it was better to seal the leak from the inside instead of just plugging the tire from the outside. While doing this, he noticed that the tire-pressure sensor casing was cracked, and he wanted to replace the entire sensor.

I told him not to do it, simply because I knew the sensor was working fine since it was the sensor that had alerted me to the leaking tire recently. Two days after the repair, the tire-pressure monitor light is blinking, indicating a malfunction in the system. This is too much of a coincidence, to me. Are the sensors that delicate, or are the mechanics that clumsy?

-- Phil

Well, first of all, your mechanic was right about sealing the tire from the inside. That's absolutely the right way to patch a tire.

Was he clumsy? It's possible. If he was really careless, and put the tire on the tire machine incorrectly, that could have broken the pressure sensor. But it's not very likely.

Was he dishonest? Did he whack the sensor by accident, and then shrug and tell you that you had a pre-existing crack? Again, it's possible. But at this point, how are you ever going to prove it? And it's just as likely that he was telling you the truth.

I think your mistake, Phil, was not agreeing to replace the sensor once you knew there was a crack in it. Now, instead of being out $75 for the sensor, you're out that much plus the cost of removing, remounting and rebalancing that same tire.

My brother used to say, "It's the stingy man who spends the most," Phil. I think he was absolutely right about that. Of course, he also used to say, "This is definitely my last marriage."

But here's a bigger issue to consider: If you trust this mechanic so little that you told him to leave a broken part inside your tire for fear of being ripped off, it's time to find a mechanic you really do trust. Get a recommendation from friends or family. Or check out www.mechanicsfiles.com for recommendations from our readers and listeners.

File this one under "lessons learned." And congratulations on your upcoming new pressure sensor and unnecessarily remounted and balanced tire, Phil.