My gas Dearborn model#drc40mc pilot light stays lit.when I turn to on position and heater comes on , it burns high , even on low number setting, then after few seconds goes out taking pilot out also

Propane and natural gas heaters have standing pilot ignition or electronic ignition. If you have a standing pilot ignition you should see if the pilot light stays on when lit and after the valve is turned to the full ON position after lighting. Electronic ignition should light the pilot for you and then the burner comes on - but at the end of the cycle the pilot and the burner go off. If you do not have ignition, check the gas pressure -- 5-7 inches of water column for natural gas and 11-15 inches of water column for propane. Use the pressure noted on the regulator or other labeling. If you cannot test the pressure then contact your gas company or propane supplier for service.

I am not a service professional, but just a homeowner who has some experience with this gas log.

GAS LOG INSTRUCTIONS

Brand: Marco Gas Log (company is now out of business)
#792775, unvented gas log
Installed late 1990's when house was built


The gas is turned on and off using the key on the wall to the left of the fireplace. The gas is OFF when the key is turned as far clockwise as possible. Turn the key counterclockwise about a half turn or whole turn (180 degrees to 360 degrees) to light the pilot and turn on the log. You don't have to worry about gas coming out into the room before you light the pilot, or if the pilot goes out.

Horizontal as far clockwise as possible = OFF
Vertical ½ turn = ON

The log needs to be cleaned before using at the beginning of the season. Remove the logs, taking note of their position so you can replace them the same way. Use a toothbrush to clean the thermocouple, which looks like two little wires immediately to the right of the pilot (which looks like a metal tube that is located in the center just in front of the back log). Use a vacuum cleaner hose and duster to clean dust off all metal parts and the fire box interior. You can go ahead and replace the back log before lighting the pilot.


HOW TO LIGHT THE PILOT:

With the gas turned on with the key on the wall, find the black and white knob on the lower right of the gas log and turn it all the way to the right (clockwise) to the to the right as far as it will go, then turn it back ever so slightly to the left until you feel a little resistance (like a click that you feel rather than hear). The pilot cannot be lit when turned all the way to the right.

Push the knob in with your thumb and use your other hand to light the pilot with a long-stem lighter or match. Hold the knob in for at least 30 seconds. The thermocouple has to be hot before the pilot will stay on. Let the knob come out very slowly. Hopefully the pilot will remain lit, but if it does not, you will need to repeat the procedure until it does. Be sure to hold the knob in until the thermocouple is hot and release the knob very carefully. When the pilot stays lit, then turn the knob to the left counterclockwise until the flame stays lit and carefully place the logs back into their original position. Note: Push the knob in only when you are actually lighting the pilot, not while you are turning the knob.

Set the thermostat from Hi to Low as pictured on the knob. Once the log is lit, it should stay on until the thermostat causes it to go off. Hopefully the pilot will stay on so that the log will come back on automatically according to the setting of the thermostat (from low to high). It will be at its hottest when the knob is all the way counterclockwise.The flame will go off and on according to the temperature in the room. The pilot should stay lit even when the main flame goes out, as shown above.

The Little Giant units use a Unitrol gas valve very similar to a water heater in the home. #1,urn the Unitrol gas valve to the "PILOT" position to ignite the pilot. Depress the plunger and hold it down then push the spark igniter button until you can see the pilot flame ignite.
You have a lit pilot so this is not a problem for you.
The pilot light stays on continuously by design.
Once the pilot flame is established, you then turn the on/off/pilot valve to the "ON" position and set the thermostat to the desired temperature range., it should then fire its main burner until the desired temperature is reached. Once the temperature is reached, the main burner will shut off and the heater will go into a standby mode with the pilot flame still burning.
If you have turned the pilot valve to on and set the temperature high, it should come on, if not a new gas valve is required, they can be very difficult to replace due to rust.and most cases require a new unit,

this happened to me as well. my pilot light had 3 settings.
"off" which is in the middle.
"up" is for priming/starting the pilot light.
"down" is what you move it to AFTER the pilot light is lit.

prior to lighting the pilot light, I had get city get city to verify gas was coming into house. It was not. someone thought they had turned gas on, but in fact did not.

so I had two problems.

If pilot lights and flame is robust and clear blue, and pilot goes out later, then some suspects can be excluded: air in gas line, low gas pressure, thermocouple is overtightened (finger-tight + 1/4 turn), thermocouple not positioned correctly in the flame, dirty pilot tube.
If flame is yellowish and small: then clean pilot tube and combustion parts.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#clean
Remaining suspects can include: air in gas line, low gas pressure, bad gas control valve, tight house so household vent fans pull air down water heater vent that blows out pilot, hot attic with inadequate ventilation, water dripping down vent.

Open following links to read troubleshoot resources.
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

If I understand correctly, the pilot light is not staying lit.
You light the pilot and water in the tank reaches temperature.
Once hot water is gone, then pilot light has to be lit again.

Gas pressure is good, and gas is flowing.
Thermostat is working at high end because it cuts off heating.
Vent is working because tank heats up.
Air intake is working.

Gas control thermostat might not be reading low temperature. Replace gas control valve.
Pilot tube might be dirty. Clean burner parts and combustion chamber, and air intake, and vent.
Backdraft might be blowing out pilot. Check for constant updraft by running water heater 5 minutes, and then light match under vent hood and see if smoke goes up.
Very hot air can put out pilot, especially when water heater located in attic. Add ventilation.
Improper vent might have water dripping back down into combustion chamber and putting out pilot light and rusting out the parts. Inspect for rusted parts and water.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf

Be sure that the gas line to the heater is ON.

Looking at the Control Knob on top of the heater, you note that he knob has 3- positions: OFF, Pilot, ON.

Turn the control knob to the Pilot position and push down on it. If it does not go all the way in and your do not feel strong pressure pushing back at you, it is not directly in the pilot position. (Reposition the knob slightly and try again). Once You're sure the knob is directly in the pilot position and the knob is depressed, continue to hold it in and press the Ignitor button next to it. You will hear a loud click. If possible, watch the pilot assembly inside the heater when you do this. That way you can see if the pilot lit the first time. If not repeat, pushing the ignitor.

If the gas line had been turned off to the heater, the pilot may not light the first couple of tries, due to air being in the gas line. If that is the case, hold the control knob in until you smell gas, let off, wait 5 minutes and repeat the lighting sequence.

Once the pilot does light, continue to hold the control knob in for 1 minute. This will allow the Thermocouple to heat up completely. Ease off the control knob and the pilot should remain lit. If it does, turn the control knob to ON and the burner will light.

Now you're ready to set the temperature or comfort level. You do this by raising or lowering the flame height. The higher the flames, the more heat it puts out. The lower the flames, the less heat it puts out. The stripe on the knob represents the flame heights - Narrow stripe, lowest setting, widest strip, highest setting. Or your specific model may simply say Low, Medium, High.

If you do not have the Owners Manual for this heater go to http://www.desatech.com/
click on Wall Heaters and enter the Model Number of your heater. You can then download and print the Owners Manual.

Hope this helped to get you up and runnig.

Turn the knob to PILOT. press the knob down. Light the pilot and hold the knob till the pilot stays lit by itself. Then advance the knob to the ON position. Turn up the thermostat.

If you need further help, reach me via phone at https://www.6ya.com/expert/dan_73bbd84fe1d95b61

sounds like the thermocouple is going out this is the part the flame from the pilot burns on and tells the gas valve to stay open by producing a small amount of electrical current in milli-amps these can be purchased @ a good hardware store and you must position as it is when it comes out , and must be the same length , make sure to connect to gas valve clean and tight , also is there a moisture trap in the line if so you might need to turn off gas and make sure it is free of moisture , this is a good place to start with these items

You may need to take the pilot assembly apart and clean the pilot orifice. If the pilot does not stay lite you may need a new thermocouple. Good Luck.