I had a similar situation shortly after replacing all my tires and though the tires were brand new they did not replace the valve stems and they had slow leaks in two of the four tires, this may be your problem. The tire pressure sensors can be very sensitive.
Tires do not have oxygen sensors. They can have pressure sensors. The lack of pressure sensors would have no affect on fuel economy.
if its in the instrument cluster, it is probably a tire pressure light, which means one of your tires is low, if it isn't then take it to a shop and they can turn it off by recalibrating the sensors.
P205/70r15 95s
The recommended tire pressure for OEM tires on a Toyota Sienna is 35 psi, but you should pay attention to and follow the tire manufacturer's specifications for any aftermarket brand of tires that you purchase.
It does not have one - the Siennas have run-flat tires.
Sienna's have two knock sensors located in the valley underneath the intake manifold. Replacement is a three hour job.
Yes, you can do it.
Yes, by installing 15" wheels.
Some brands of cars that have car reversing sensors include Toyota and Dodge. For example, the Toyota Sienna has sensors that alert the driver when there is an object directly behind the rear bumper.
can you safely drive a 2000 camery with bad sensors
No, it did not come with them 10 years ago when it was new.