I installed a set of aftermarket wheels on my '06 GS300 and didn't get any TPMS warnings for months either. That made me wonder about the whole TPMS sytem too.
The only two times the system indicated a TPMS message were both on longer distance trips. The first one was 65 mile drive and the second one was on a 300 mile trip. The rest of my driving is around town and therefore not much time or distance. It seems to me the system needs to have the error condition present for a longer period of driving time before it indicates anything is wrong (or in our cases, that there are no TPMS sensors even present). If the same is true of low tire pressure conditions when there are sensors on the tires then I'm wondering how helpful the whole TPMS system is if you really did have a tire with low pressure.
BTW - I did buy a set of 4 used OEM sensors (Toyota P/N 42607-30030) on ebay for $100 and had them programmed at the Lexus dealer for $52 to get the system back in order for communicating with the extra set of wheels. The first set of wheels with the origianl factory sensors now has snow tires mounted. One nice thing about the GS TPMS system is that it has capability for two separate sets of wheels and you can switch back and forth between the two sets conveniently.
