If the problem with your Whirlpool 29 inch gas dryer is that is not heating at all, or it heats for 20 minutes then is not heating anymore. The problem could be bad gas valve coils.
Taking a reading with a multimeter when the coils a cold, could show that the coils are good. But after the dryer heats for 10 to 30 minutes they fail to open the gas valve.
When I check a gas dryer, this is what I look for.
I turn on the dryer on the regular cycle
If the dryer has a peep hole to check the gas valve, I take a look through it
If the igniter glows and the gas valve releases the gas, and the gas ignites
I let it run for 30 minutes, then check it again
If the igniter glows, but the gas valve doesn’t open to release the gas
I go a head and replace the coils on the gas valve. Most of the times that takes care of the problem.
If that doesn't take care of the problem, most likely the gas valve is bad and most be replace.
Below are the links to the parts needed for this repair.
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