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WARNING LIGHTS
Chevrolet tire pressure monitor system sensor. These are inside all 4 tires and if go bad will cause the Service Tire Monitor System warning light.
This is a typical tire pressure sensor on GM vehicles. This is used
on TPM systems in your Chevrolet, Buick, Pontiac, Cadillac, GMC
and Oldsmobile cars and trucks.

There a differences between this part depending on model and
year. Other sensors have a black stem. These must be pulled
through the rim with the tire removed.

The picture above is the type with the nut on the outside of the rim.
The tire still needs to be removed from the rim to replace.