The following process will allow you to test the thermal fuse and replace it if you find it open.
Unplug unit and turn off gas if gas model. Check to be sure you have 240 Volts at wall outlet.
Most parts likely to fail on electric dryers are inside the rear cover. Heater is on right inside can, safety thermostat on lower left of can, High limit thermostat (non-resettable) higher left of can. On the blower housing to the left are the operating thermostat and, in some models, a thermal fuse (white plastic). ALL can be continuity checked easily. Always disconnect wire on one side of what you are testing. One will likely be open. Replace the component that fails the continuity test.
Thermal fuse 3399849 trips if the exhaust air leaving the dryer becomes too hot. 
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Thanks, Controlfrk. I have a Kenmore 110.96551220 electric dryer that won't heat up, and your trouble-shooting comments were helpful. With an ohmmeter, I get infinite resistance on what looks like a fuse or thermostat on the upper left side of the heater can. I believe this would be what you are calling the high-limit nonresettable thermostat. The Kenmore exploded-parts diagram that came with my dryer seems to show it as a "Thermal Cut-Off, part no. 3399848" But when I go to various internet parts suppliers, including Sears, and look up that part number, the pictures posted by the vendors on their sites look different from the part that is actually in my machine. The part that I have in the dryer is a little white boxy thing with two prongs, and what is shown as being available for replacement is a round thingy (see thishttp://www.partselect.com/PartDetail.asp... Do you think that what is being sold under that part number is the right part, and maybe the appearance of it has simply been changed in the 16 years since this dryer was manufactured? Or am I completely on the wrong track, and maybe I have the wrong part number altogether? The owner's parts diagram shows this as the only thermostat, fuse, or cutoff-type part in the vicinity of the actual dead part that I tested. So if my little white boxy thing with the infinite resistance is not part no. 3399848, I don't know what it could be. I really would like to get my dryer going again.
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