There is a pre-determined threshold for low tire pressue. If any tire pressure is lower than that limit, it will light up the warning light. And the TPMS only doing the measurement when the car starts moving and before it reachs 20mph IIRC. A lot of ppl, me included, got the warning light this past winter. Somehow I have a feeling that I have to get used to that warning light every winter
Luckily, Subaru didn't put a senor in spare tire. My friend had to take his spares out from his 4Runner ( from the same spot as Tribeca's spare) and put in some air because it is the one that gives out warning.