Scion Frs rims on 2007 Prius TPMS "low pressure warning"

what pressures are you running?

 

I may be wrong but I thought you had to have the new TPMS sensors programmed to your car. How you made it 3 months w/o the lights I don't know (unless you had your old wheels in the back for the past 3 months).

 

Lack of TPMS warning light for 3months is likely due to the following:
1) You still have the original Prius wheels w/ TPMS, and all four of these tires are inflated properly, and stored in the garage where you park the Prius w/ the current Scion wheels and Scion TPMS.
2) The Prius has been getting TPMS readings from the original Prius wheels each morning. Also, your drives have been very short, which would explain the lack of TPMS not getting triggered much sooner.
3) A sensor or sensors from the original Prius wheel/s has/have gone bad, OR, one of the original Prius wheels/tire has developed a leak in relation to the other three Prius wheels/tires.
4) Given your 2007 model year, the TPMS in the original Prius wheels are most likely dead. The batteries in TPMS seem to last 7-10 years; some people will get a few years less, some a few years more.
5) If you want TPMS to function correctly, the TPMS in the Scion wheels have to get programed to the Prius. Call you local tire stores to see how much the will charge for this. It will likely be MOST cost effective to get new sensors programmed on the Scion wheels when you need new tires.
6) There is a VERY REMOTE chance that the four Scion TPMS has the same ID #s as the original Prius TPMS. But this would be like lotto odds.
7) You also have the option of just living w/ the light on or putting tape (electrical) over the TPMS warning light. However, some States will NOT allow any warning light to be on during inspection for registration renewal; check your State's requirement(s).