why does my check tire pressure light come on and stay on?

Some cars (Lexus and Toyota)that use a full size spare wheel also monitor the spare wheel pressure and will set a light if the spare is low. It is also possible that there is a fault with one of the sensors at the wheel (they do after all have a battery not much unlike a watch battery at each wheel sensor), but it may be necessary to have the system looked at using a scan tool to locate the fault. Are there any reset procedures mentioned in your owner's manual? Check here http://repairpal.com/automobile-owners-manuals-online or try this link you may be able to down load a free copy of the owner's manual for your vehicle http://www.autousermanual.com/