So the light went on, and said left rear was 15 psi. I wanted to check all of them anyway, and did. I have a fancy one in the work truck, with a short hose and a gauge. LR was actually around 28 psi, not far off the recommended 33, which the other 3 were right on. May have a nail in it, I did add air on Sunday. I guess I need to understand where the sensor is, how it works, and if it can be replaced.

Is this on the 1998 Bonneville listed in your profile?

No, the 2011 Lacrosse.

The sensor is in the wheel. It is battery powered. It can be replaced by any normal tire shop.

Thanks, CC!

UPDATE: On the dash, it'* saying left rear, but it'* actually the right rear that was low! Down to 3psi, and I took it on the highway! So, I filled it when I got home, but why is it saying it'* the wrong wheel? Is there a sequence for rotating tires?

Someone rotated tires and didn't tell the BCM where they ended up.

Sensor IS the same as where you put the air in, they have to break down the tire to change them. Battery powered but no replaceable batteries . . . Tire store or dealer..

UPDATE: I aired it up last night, and back to 10psi this morning, so I took it to Mr. Tire, it was leaking at the bead! Not too common. The rim isn't dented or cracked, so they put some bead sealer on, and reprogrammed the monitors. Back in business!

Cool!

Thanks for letting us know how it turned out!

Did you have them rearrange which tire is which in the BCM'* little brain?