Expectations when setting an AC grill's temperature to HIGH.
The HIGH setting on an AC grill controller runs the grill at full capacity. Under ideal circumstances, HIGH typically places the grill at 450°F/232°C, which is also what the controller will display when turning the dial to HIGH.
While AC grills on HIGH tend to level out between 425°F and 450°F/203°C and 232°C, this can fluctuate depending on external variables such as cold or hot weather and if the grill is located in the sun or in the shade. The HIGH setting is functioning as designed if when it reaches any temperature above 400°F/204°C.