Furance Starts up and runs for Approx 15 minutes then shuts off before reaching the thermostst temperature setting. After the furance shuts off I can restart it, but each time the furance runs, it runs for a shorter time. I know it's not the thermostat and it has an electronic ignition. Does this furance have a high limit temperature sensor that may be bad? If it does have this sensor, where is it located?

All registers open and not blocked?
All return airs open and not blocked?
Air filter clean?

Check this first.