So if you have a new auto with the TPMS system will should work for you also. This does not apply only to the Yaris.
Quick back ground:
The TPMS system works on sensing pressure from the 4 tires (maybe 5 if you have a full size spare-- I don't know). Each sensor at the wheel only send data to the computer in the car, it is the car's computer that takes in info and then figures out if it should tell the driver in the form of a light on the dash.
There are more sophisticated TPMS systems which may tell you the exact pressure each tire is at, this DIY is not directed towards those systems. The particular kind of TPMS on lower end cars like the Yaris is crude in comparison to what was already state. The Yaris TPMS can be summarized to : Computer takes in info, Computer compares the pressures, Computer says "hey there is a difference" thus it lights up the dash. I believe that there is a low point where the light must go on but it seems it is set so low that you can obviously see that the tires are flat.
REASON: This is for you if you have multiple sets of wheels a la winter and rest of the year. Those sensors cost upwards of 100 bucks, plus the hassle of programming. EVen if you could afford multiple sets of sensors, it is not a matter of bolting on and go, I believe the computers only allow 4 sensors at any time. So you would still need to keep pertinent data like serial numbers or part numbers and drive down to the tire shop or dealer to re program the computer each time you switch sets.
1. Gather parts.
BITS (7/16 and two smaller for pilot holes)
Bent nose pliers
PVC Pipe (3.5inch Diameter)
ABS Cement (pipe glue)
---Total about 20 bucks (piping and glue)
2. Lay everything out to get organized:
Diameter was 3.5in. Full circumference was about 11in. 11in. divided by 4 needed holes makes about 2.75 inch spacing. (adjust spacing appropriately according to the diameter)
Inside the caps is a groove that will prevent the pipe from bottoming out. Measure to it and park on the pipe.
[ <------to groove----->][<----center for holes----->][<----to groove_----->]
Doesn't have to be exact, just make sure you have room in the center for the holes and that it's wide enough for the width of the body of the sensor..
Drill the holes, use pilot holes to make drilling easier.
5.Test fit one sensor just to make sure the hole is okay. Doesn't need to be snug. You'll need a bit of looseness for later. Take it off.
7. Put in the sensors. The first three will be okay, but the fourth will be a tight fit. This is the reason why you should not put in the nuts on any of them yet and why you cut the pipe. A bit of a puzzle literally.
An alternative arrangement is three in the middle and one on top of the cap. (do measurements accordingly)
8. Dry fit the end caps and then apply glue to both the end of the pipe and the inside of the cap. (Do one end at a time)
9. Wait fifteen minutes for the cement to set then pump air in. Take it to the car and you should be okay after it recognizes the signals.
You can vary the idea to fit your needs, this is just one example.