Kenmore 417.81142000 front loader dryer, temps off, not hot enough.

All driers will do that when the lint builds up like concrete on the inside of the air ducts. You need to totally tear it down and scrape out and wash all of the parts except the motor (washing the parts out is an extremely important step not to take lightly), which you vacuum out. Halfway doing it will not work. This includes the exhaust vent, all the way, even the outlet on the side of the house, if you have one. You can go to YouTube to see videos on how to take it apart, but do a very good job or it will fail again quickly. I would get a new belt and change it while you have it apart and get some white lithium grease and grease any bearings you find. You need to pull the support rollers apart to grease the pins with white lithium grease. You need to clean out a drier every year, at least, according to the manual. All sensors work together by comparing temperature, so all need to be cleaned. You just unplug the wires from the sensors and remove ductwork that they are attached on (Label wires!) and clean the entire thing with soap and water. Make sure you go all through the dryer and get all parts like the blower fan and housing, vacuum out motor well and clean exhaust. Mark all wires you unplug so you know where they go. This is an easier job than you would think. You need a meter to test any fuses that may be blown. You will see them on the schematic diagram.
See: Dryer vent cleaning prevents fires - YouTube (http://www.youtube.com/watch?v=A6wmD502-xE) and: Cleaning out a dryer with top lint cleanout - YouTube (http://www.youtube.com/watch?v=OBNDbyRm68M&feature=fvsr)
Cleaning out a dryer: By a woman 2 of 2 - YouTube (http://www.youtube.com/watch?v=a73HTFkcB90&feature=relmfu)
Manuals: https://dl.dropboxusercontent.com/u/8010163/Whirlpool%20Dryers.zip
https://dl.dropboxusercontent.com/u/8010163/How%20to%20use%20Test%20Equipment%20Service%20Manu al.pdf
You may want to also check any sensors which you will find where the air enters the drum and where it returns from the drum. There are charts which give the ohms and the temperatures because each sensor has a specific resistance, measured in ohms, and often they can be off in the newer dryers. If the chart says that at 70°F you should have 150 ohms and you have 200-250 then you have a sensor which will cut off the heat before the temp gets high enough. The resistance will usually go high on a thermistor (which is a resistor affected by heat in a specific graduated way- THERM-resISTOR). On older dyers you have two or more thermostats to control heat, one a little higher than the others. They work by shutting down the heat when there is very little temperature difference between the supply and the return air. These will just open when bad or will not work at the right temp. It's rare for them to go bad, but you can test them with the oven and an accurate thermometer and then use a long set of wires (heat resistant) to test with a good digital meter. Since most troubleshooting on any appliance is done with a meter, you need a good rugged digital multimeter.