04 Yukon Check Tire Pressure light

I have this vehicle with 37k miles and the original tires. The tires have been rotated and are still in good shape. The 'Check Tire Pressure" light has come on before and refilling all the tires to 35psi
had turned the light off. Today, after filling all the tires, the light stayed on. I rechecked the psi and it is OK. Anyone know how these work? Being out of warranty, is it something I can tackle?
Thanks in advance.
Karl
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Check the spare and see if it has a sensor also, if it does check the pressure in the spare.

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