Gas dryer starts to heat but shuts off while drum continues but doesn't heat or dry

Coil is probably bad, they get a crack in them and will not work. If you have a an ohm meter you can disconnect power and check it

In some dryer's the control panel relies on a thermistor rather than a CYCLING thermostat to regulate the drum's air temperature by monitoring the component's resistance changes; resistance goes down as temperature increases and up when temperature decreases. Once the drum's air temperature reaches a certain level required to dry clothes, the control panel shuts off the heater. The panel will turn the heater on again and begin another heating cycle when the thermistor indicates that more heat is needed to keep the air temperature constant inside the drum

Lastly check your moister sensor. ( located inside the dryer door usually) Especially if machine seems to shut down early and clothes are still wet.
Test with a meter at room temperature and it should show continuity.
A failed moisture sensor will affect the dryer run time in the automatic moisture sensing cycle but it will not affect the heating of the dryer or the timed cycle. Which are reflected by the thermostats.

Read more :http://www.ehow.com/info_12203962_check-dryer-thermistor.html

FIRST OFF IN THE AUTO DRY MODE THE DRYER USES THE MOISTER SENSOR AND AS LONG AS THE MOISTER SENSOR DOES NOT DETECT HEAT OR DRIED CLOTHES IT WILL CONTINUE TO RUN FOREVER.

NEXT AS FOR UR NO HEAT:

ELECTRIC Dryer no heat or little heat, or shuts down to fast:

Check your venting and lint basket. Check blower for lint build up and blower wheel obstruction., test by trying to turn the wheel manually by hand (should be easy) May have to remove cabinet or front/back plate to get to it)

Next check the heating element itself with a meter for continuity OHMS CLOSED CIRCUIT. If not its defective or has a short if its grounding out? Which in turns causes blown fuses or thermostats or overheating.

Check dryer Terminal block prongs both outside prongs should give combined 220, and 110 each if u check 1 outside & 1 center (ground) prong. Also check house electrical outlet for full voltage. 220 because if u only get half or 110 volts you will be able to run the machine which uses only 110 to run motor but not the heater which uses a full 220,

Check the thermal cut off, the cycling and the hi limit thermostats.
For continuity or OHMS. If no ohms or resistance they need replacement.

In some dryer's the control panel relies on a thermistor rather than a CYCLING thermostat to regulate the drum's air temperature by monitoring the component's resistance changes; resistance goes down as temperature increases and up when temperature decreases. Once the drum's air temperature reaches a certain level required to dry clothes, the control panel shuts off the heater. The panel will turn the heater on again and begin another heating cycle when the thermistor indicates that more heat is needed to keep the air temperature constant inside the drum

Lastly check your moister sensor. ( located inside the dryer door usually) Especially if machine seems to shut down early and clothes are still wet.
Test with a meter at room temperature and it should show continuity.
A failed moisture sensor will affect the dryer run time in the automatic moisture sensing cycle but it will not affect the heating of the dryer or the timed cycle. Which are reflected by the thermostats.

Read more :http://www.ehow.com/info_12203962_check-dryer-thermistor.html

Gas Dryer no heat: or shuts down soon after heating CHECK:

Thermal Fuse
If the air flow in the dryer is restricted, the temperature in the heating chamber can get hot enough to blow the thermal fuse. If the fuse blows, it cuts power to the coils that control the gas valve. The fuse is usually mounted to the exhaust duct just inside the back panel. You can check its continuity with an ohmmeter. If, after disconnecting the fuse, you get any reading other than 0 when you touch the leads of the meter to its terminals, it has blown. There's no way to restore it -- you'll have to install a replacement.

Igniter and Gas Valve Coils
TEST with a multi meter: The igniter is an electric conductor that works like the element in an electric heater, glowing hot enough to ignite gas when you turn on the dryer. This conductor can burn out, and when it does, it may glow, but it won't get hot enough to ignite the gas. At times it may give a reading of continuity yet fail mechanically and not get hot enough to ignite flame though it gets hot and glows and even may show Ohms or continuity ( close circuit) ?
Occasionally, the Gas safety valve and the electric coils that control the gas valve are defective -- they can wear out when the dryer gets old. When this happens, the igniter glows, but gas never enters the heating chamber or does not stay consistant.
Performing a continuity test on either part will help you determine whether or not you need to replace it. But the coils should also be tested for amount of resistance as well as Ohms. As they may be showing ohms yet not putting out enough resistance to keep proper gas pressure flowing. Most coils should show at least 1300 ohms ( GIVE OR TAKE 150 OHMS). Anything significantly less Thus u get an ignition but then it soon goes back out. The flame does not stay lit.

Air Flow and Heat
The motor that drives the tumbler also drives a fan that circulates air through the heating chamber and the tumbler and expels it through the vent. If the air can't circulate, perhaps because of lint blockage, the heating chamber overheats, which prompts the cycling thermostat to turn off the gas. The thermostat resets when the chamber cools, but the chamber heats up quickly and the thermostat again shuts off the gas. The result is that the temperature in the tumbler doesn't stay hot, and your clothes take longer to dry, if they get dry at all.

Warning
There's a big difference between a dryer that doesn't heat up at all and one that just doesn't get hot enough to dry your clothes. In the first instance, the problem is usually a defective part, and things should be back to normal after you replace it. In the second instance, the problem is caused by restricted air flow, and you need to clear the lint filter and vents and take steps to prevent lint build-up. If you don't, you'll use more energy for drying than you need and the dryer may continue to malfunction. Worse, you may have a dryer fire.

Hello there and welcome to fixya there are several things that could make this dryer not heat up
Here are the most common things

Thermal Fuse

If the dryer doesn\'t heat, but the drum
turns, check the thermal fuse(s). The thermal fuse protects the dryer
from overheating and helps to prevent fires. If the thermal fuse is
blown, it will have to be replaced.




Enter your model number to see parts for your dryer.

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Gas Valve Solenoid

The gas valve has two or more electric
solenoids which open the valve to let the gas flow into the burner
assembly. If the dryer doesn\'t heat one or more of the solenoids may be
defective. If the igniter glows for 90 seconds but the burner flame
doesn\'t light, replace these coils as a set.


Dryer Igniter


If the dryer doesn\'t heat, the igniter might
be burned out. Check the igniter for continuity with an ohm meter. Dryer
igniters burn out over time, similar to a light bulb.

Heating Element

The heating element on an electric dryer can
burn out over time. If the dryer doesn\'t heat, the element should be
checked for continuity. The heating element cannot be repaired and must
be replaced if defective.

Heating Element Assembly

the heating element assembly on an electric
dryer can burn out over time. If the dryer doesn\'t heat, the element
assembly should be checked for continuity. The heating element assembly
cannot be repaired and must be replaced if defective.

Flame Sensor



Gas dryers have a flame sensor to detect the
heat given off by the flame. If the dryer doesn\'t heat, the flame sensor
might be defective. Check the flame sensor for continuity. This is not
as common as a burned out igniter or thermal fuse.


High Limit Thermostat


Although not common, if the dryer doesn\'t
heat the high limit thermostat can be defective. It can be checked for
continuity. If the thermostat is "open" the dryer doesn\'t heat.


Cycling Thermostat




The cycling thermostat regulates the
temperature of the air in the dryer. If the dryer doesn\'t heat it may be
caused by a defective cycling thermostat. This is not common.


Main Control Board

The main control board is normally not at
fault when the dryer doesn\'t heat. Check all of the other components in
this troubleshooting guide before replacing the main control board. The
main control board cannot be tested easily and must be replaced if it is
defective.


Timer
The timer is normally not at fault if the
dryer doesn\'t heat. In many situations where timers are replaced they
end up being returned. Check all of the more common components in this
troubleshooting guide before replacing the timer

I believe you will find the belt is broken.
There is a motor which is supposed to turn a belt, which in turn drives the drum.
If you wish to repair ir yourself, a letter to "Fixya" could cause someone to walk you through the steps.
Be blessed.

See if you can spin the drum with little resistance or is it really hard to spin?

1st go the the house breaker (dbl breaker) for the dryer and shut it off and turn it back on, your dryer runs on 220 volts of power but if the breaker is partially tripped it may be only running on 110 volts which is only enough power to turn the lights on and to turn the drum the element needs 220 volts to turn on. If it still doesn't heat after resetting the breaker then you may need a heating element or thermostat. The dryer isn't shutting off because it isn't detecting the clothes are getting dry so once you fix your heating problem the dryer should start shutting off on its own again.

Hello,
A very typical problem with GAS dryers is the soloniod coils that open the gas valve . When the coils are cool they operate just fine,but once the dryer gets warm from running the coils dont open the gas valve wwhen they are warm. If that is what your experiencing,the dryer heats when its cool but doesn't heat when it's been running for a while,the coils on the gas valve will need to be replaced.

GENE

If electric then the thermostat or the heating element has failed.
If gas: First thing to check is the ignitor underneath the drum. (You have to open the panel to see it) It should glow shortly after the dryer starts, then the flame starts and the ignitor shuts off. If the ignitor does not glow then it or the thermostat is bad. If the ignitor is glowing but the gas does not light then the flame sensor may have to be replaced. If the gas ignites but does not stay on either the flame sensor or the coils are at fault.
See also: http://home.howstuffworks.com/how-to-repair-a-dryer.htm