Check out the heater element.
Remove both wires from it then check it for resistance usually around 10 ohms.
Then check each terminal to the case/frame should be infinite (open).
In not then you have a grounded element which can cause it to run unregulated. Could be it is broken and touching the case or may have sagged and touching the case.
You can heat up the thermostats on a stove top element to see if they open.
Usually you can hear them click and then check them for resistance.
If not their internal contacts are welded together.
Be careful not to burn yourself.