I would first check to see if the heating element is shorted out. When the vent or blower housing of the dryer get built up with a lot of lint the dryer won't vent out the heat properly and it can cause the heating element to warp and short out against the cabinet. To check for a shorted element you should unplug the dryer and remove the bottom panel. On the right side you will see the heating element, remove the two wires that connect to the element. With a multi-meter, check to see if you have continuity from either side of the element to the cabinet that hold the element. You should NOT have continuity when you check for this, if you do the element is shorted out and will need to be replaced. You can also pull out the element and do a visual inspection, sometimes you can see where the element coil is touching the housing. When the element is shorted out, it will continue to heat when the dryer isn't calling for heat. They will even heat with the dryer not running, so make sure to unplug it for safety reasons.