My siemens oven is only heating on the highest temperatur...how come??

SOURCE: Why won't my oven heat to the proper temperature?

did you mease the temp = how do you know its wrong

your old stove might ahve been too hot

SOURCE: I was baking something in

The oven control switch has burnt shut. I have replaced several for this cause.
What occurs is the switch opens and closes by pressure on a small diaphragm which
activates the switch contacts. The little copper bulb sensor in the oven produces this
pressure. If the contacts burn or stick then the bulb cannot regulate and the oven stays
on. These are pretty pricey, usually in the 40 to 70 dollar range.
The model and serial number should be visible if you slide the drawer out and look
on the left side. Usually all that has to be done is removal of about 6 screws to
remove the control panel depending on manufacturer.
Be sure the unit is unplugged before servicing.
Hope this helps.