keep getting tire pressure warning yellow light checked tires several times, on 2009 Mercedes-Benz C300 4MATIC

Could my sensor be broken or prossible losing air in one tire; what to do; added air and all tires are correct problem just started 6/28/13; how to do tell which tire

Asked by for the 2009 Mercedes-Benz C300 4MATIC
Checked the spare tire also?
This is a useless site, when you get of no use replies; why check the spare tire that hasn't been used. This site I will unsubscribe to.
THANK YOU!! It really does seem useless, from here!! FYI: If the spare tire has lost pressure, as all tires eventually do, it will make that yellow light come on! Info. can be found in the owners manual. Better yet go to a Benz Dealer, which will be more than happy to check it for you.

Ps, now go slap a Polar Bear 3 times for good luck!!!