There's batteries in them so they do eventually die just like any battery. They usually last 5 to 10 years so at 7 years yours could be on the way out. Usually a couple hundred bucks to replace. Parts cost is about half of that, the other have is to pull the tire off the rim, replace, re mount and balance. Pretty much every tire shop you buy tires from can replace them. Any reputable shop should tell you over the phone exactly how much they charge so look up some in your area and make a few calls.
You could just ignore it since you checked the pressure yourself and you know you they're good or replace them, its really up to you.

 

Can someone post the part number for the 2015 Toyota sienna TPMS sensors, please....

 

Try Toyota Parts and Accessories - Official Online Store

 

I am thinking it is this one, but you can verify by entering your VIN:

2015 Toyota Sienna XLE 2WD Tire Pressure Monitoring System (TPMS) Sensor. ALLOY, BRAKE-DISK - 426070C070 - Genuine Toyota Part

Tire Pressure Monitoring System (TPMS) Sensor. ALLOY, BRAKE-DISK. 2015 Toyota Sienna XLE 2WD. Genuine Toyota Part - 426070C070 (42607-0C070, 4260708010, 426070C030, 426070C050, PT5863408B)

parts.toyota.com

 

The TPMS system will throw errors codes that tell the problem. If a sensor battery goes dead, then the TPMS light will blink for a minute, and then go solid, and a code will be stored, "signal not received."

So does the OP's light go on solid, or does it blink for a minute, first?

 

Solid light.

Javvy

 

Then the sensors are sending their signals.

If you don't have a TPMS scan tool (like an Autel product, for example), many tire shops will read your TPMS sensors for no charge (Discount Tire is where I'd go, if there is one nearby).

It is possible your tire pressure gauge is not accurate. Have you more than one? One that I had, for years, read 15% low, I eventually discovered!

 

I have a very expensive tire pressure gauge, I am aware of the cheap Chinese make Guage discrepancies and that is the reason why I bought a really expensive manual guage that I can adjust to 1pse increments. I will swing by a tire shop get the TPMS sensors scanned and find out what going on. Thank you for the advice.

Thank You
Javvy