I'll start by saying this isn't a Prius only problem but I discovered it on mine today. I guess it dates to the Ford / Bridgestone tire underinflation fiasco.

I've had a snow leak in one of my tires all winter. I chalked it up to the extreme temperature swings we have had (-15F to 50F twice between November & February). I ended up getting a nail in the tire yesterday & took it in to get fixed.

The tire was an easy fix, but the service tech said the valve stem was bent (which explains the slow leak). He said to fix it required replacing the stem and the site pressure sensor inside the wheel. He said to replace the valve stem and sensor was $140.

I looked at him and said, "Can't you just replace the stem?" He said he'd have to ask the service manager. He left for about 15 minutes.

When he came back, he said, "We can replace the stem and sensor for $140 or replace just the stem for free and the sensor light will be on all the time."

I said, "I can live with the sensor light being on all the time."

So the did it. And when I left the shop, the low air pressure light wasn't even on.