First, George, I would ask you how you determined in needs a Thermocouple?

Is the Pilot Light, lighting and holding?

Does it go out when you turn the Control Knob to ON?

Does everything go out completely after a minute or two?

If you answered yes to any or all of the above questions and your heater is no more than 6 years old and hasn't been cleaned and serviced annually, it's probably a case of the Oxygen Depletion Sensor (ODS) being clogged with dust, lint, spider web or pet air. Therefore, the ODS reads the oxygen level in the room, as unsafe for operation and doesn't allow it to light and hold.

You can clean it yourself by removing the front half of the heater cabinet after turning the gas off to the heater.

You'll need a vacuum cleaner that has a hose and brush attachment (a shop vac works real well).
a can of compressed air with a straw nozzle (like you clean your computer with) and a flashlight.

Once the front half of the cabinet has been removed and set off to the side. You will have access to all the inner workings of the heater. Plus, if it hasn't been serviced annually, you find a lot of dust and lint.

Using the vacuum with a hose and brush attachment, vacuum all of the interior (including the burner(s) that you can safely reach. If your heater has an optional blower, vacuum that too.

Once the vacuuming is complete, take the can of compressed air with the straw nozzle attached and blow out the pilot tip where the flame comes out. With a flashlight, look down the pilot assembly from the pilot tip and about 2/3rds of the way down you will see a small hole in the side of the pilot assembly. That's the ODS sensor. Place the tip of the straw nozzle just barley into that hole and hive it a quick blast of air. If you watch closely, you will see debris fly out the other side.

Now, take a look at the Thermocouple (by the way, it's not a "Thermocoupler"). If it's a stand-a-lone thermocouple (Brass rod that sticks up in front of the pilot tip), does it look ashy or white? If so, take a strip of very fine sandpaper or emery paper and polish the thermocouple. You don't want to sand it down ... just polish it. After completing that, take a cotton ball dipped in alcohol and wipe the thermocouple tip off.

Before reassembling the cabinet, turn the gas back on and try lighting the pilot. It probably won't light the first time, because there's air in line from the gas line shutoff valve to the heater. If it doesn't relight immediately, hold the control knob down in the Pilot Position until you smell gas. Let off and wait 5 minutes, before trying to light the pilot.

If the pilot lights and hold after you have held the Control Knob in for one minute. Ease of the control knob and turn the knob to the ON position. The burner should light. Take note of the flames and their color. They should be Blue with Yellow tips. And the burner flames should be coming from all the little burner ports (holes).

If it all checks out well, turn the heater off, allow it to cool and reassemble the cabinet.

If however, none of the above works, then you will need a Thermocouple. You can order it online from http://www.desaparts.com/

You will need your Model Number and Serial Number.

If your Thermocouple is not a stand-a-lone Thermocouple, you will have to order a complete Pilot Assembly.

Good luck! Please let me know if cleaning the unit solved your problem or not.

Rich