We have an older Whirlpool driver Model LER7646JO0 that is not heating up. We've cleaned out the lint trap and the exhaust tube, but still no heat.

Anyone with ideas on what to do next? Is it possible the heating element just needs replacing? If so, is this a D-I-Y project?

Could be just the high limit safety. It is located on the vent at the heat box. $10 part but you need to test before replacing parts. Have a VOM? Safety and heat strips are DIY.

VOM = Voltage Meter? I have one of those. I removed the vent from the back of the dryer. Strong airflow coming from dryer, but cool air.

Look on the heater box for a small, usually oval, sensor with 2 wires. Briefly connect the 2 wires and start the dryer. If it works, the safety is bad. Next, use the VOM to test the coils in the heater box. Power off to dryer, VOM to resistance. If the meter shows infinity, you have a bad coil or strip.

Not to sound dumb but where do I find the heater box?

Inside the dryer case, it contains the heater coils or strips. Need to remove the back.
Go to You Tube and search videos for "replacing high limit safety on dryer" and "replacing dryer heater coils".

Perfect. I found a video that looks exactly like my model. I'll let you know how it works out.

Most likely it is dirty (The little bit of cleaning you described did nothing.) and lint deposits have built up, as it always does, in all the air ducts which will cause the main safety to blow causing the problem you described. The thermal fuse is behind the drum and positioned so that you will have to take the dryer apart to get to it. There is usually more than one fuse. You need a multi-meter to find which safety has blown. You will have to take it all apart and clean the drier out thoroughly or it is all for nothing. Every air duct will have to be removed and scraped and washed out to remove baked on lint deposits which bakes on like a concrete lining. This will burn so the manufacturer has installed the safeties to keep from having a house fire. The safeties are oval-shaped with a round center part containing the safety with two plug-on terminals for the wires and when they burn open, from the dryer being dirty and overheating, nothing will work, or it will not heat. Using an ohm meter you put a lead on each terminal of the safety, with one wire unplugged, and if it is good you will have a reading of about zero; if it is bad it will read infinity. On your dryer you may also have to test some thermistors if it has them. They are sensors that are around in different places to test the temperature and you test them with a meter set to ohms and compare the reading to a chart. They are specific to the dryer, but if you read nothing or a short you will know it is bad. Probably, once you clean it the dryer will run find after the blown safeties are replaced, too. Driers were the number one cause of fires at one time, hence now we have safety fuses. Look at: How to clean your clothes dryer. How to make it hot again. - YouTube (http://www.youtube.com/watch?v=MMP7BW1lLs4) to see how. Also look at these to get an idea about how to clean it: Mr. Appliance Tips: How to Clean Your Dryer - YouTube (http://www.youtube.com/watch?v=iBiRw-KElC4) ; Cleaning a Dryer Vent and Lint Trap - YouTube (http://www.youtube.com/watch?v=yFTZGgVisCI)
Electric Clothes Dryers (http://www.electrical-forensics.com/Dryer/ElectricClothesDryers.html) 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)
When you finish cleaning the drier it is best to also replace the belt, too, because you do not want to have to disassemble it all over again and if the belt is over two years old you should replace it, and you will have to grease all bearings with white lithium hi-temp grease. Here are some manuals: https://dl.dropboxusercontent.com/u/8010163/Whirlpool%20Dryers.zip
Remember: Anything you do half-way will have to be re-done as it only takes one section with lint build-up to cause it to fail again. Don't neglect the blower and housing or the section where the lint screen is located, either.

Need some more help, see https://www.askmehelpdesk.com/electrical-lighting/micronta-multi-meter-790569.html#post3644447

Well the dryer is blowing hot air again. I changed out just the high level thermostat because the other one was a slightly different model that required a jumper cable which meant cutting some wires. Turned it on and it immediately started blowing warm air which got hot real quick.

My wife is a happy camper.

I hope you fixed the cause of the safety blowing because if you didn't you can burn your house down, and by the statement that it got hot fast it leads to believe that you created a fire waiting to start. Any time a safety blows, especially the high limit it is a sign of extreme danger of fire because they do not blow easily.

Yes I cleaned out the dryer and no it didn't heat that fast, just what would be expected.

The exhaust vent to the outside was completely blocked with lint, which is what I expected caused the problem in the first place.

Sometimes I have seen the exhaust vents have screen over the opening which catches lint rapidly; also, I have seen the duct work installed with rough edges that tend to catch lint and slow down airflow. Dryers should be cleaned on a regular basis according to how much it is used. If it's in a house with kids then you would need to clean it at least once a year. At one time dryers were the number one cause of house fires.

I fully understand. There are no children in the house, the dryer is used basically one day a week with 2-3 loads. I was very remiss in cleaning the ductwork, something I will rectify in the future.