Will light ,but keeps shutting
Form what you have described it sound like the Oxygen Depletion Sensor(ODS) is clogged with dust, lint and/or pet hair. 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.
I hope this helps to solve your problem.
Rich