Turn off the main gas line that feeds the thermostat and turn the thermostat to the "OFF" position. At the bottom of the water heater, remove the outer door that covers the thermocouple, manifold tube and pilot tube. This exposes the manifold door.
Remove the two screws that hold the manifold door to the water heater. Use an adjustable wrench and remove the thermocouple, manifold tube and pilot tube from the bottom of the thermostat. Turn the thermocouple clockwise to loosen; turn the manifold tube and pilot tube counterclockwise. Pull the tubes and thermocouple from the thermostat.
Slide the manifold and burner assembly from the combustion chamber. Use a vacuum to pick up any dust and loose debris from the combustion chamber. Use compressed air to blow the dust from the flame trap located inside the bottom of the combustion chamber.
Check the ends of the pilot and the thermocouple connected to the burner assembly for carbon build-up. Carbon build-up will appear as white on the ends of the thermocouple and pilot. Use a soft wire brush to brush away any carbon build-up.
Replace the thermocouple if kinked or bent. Pull the old thermocouple from the burner assembly and push the new thermocouple into the opening. Position the end of the thermocouple over the pilot flame opening. Replace the manifold assembly into the combustion chamber.
Reconnect the thermocouple, manifold tube and pilot tube to the bottom of the thermostat. Tighten the thermocouple by turning counterclockwise and the manifold tube and pilot tube clockwise.
Attach the manifold assembly to the water heater with the two screws you removed in Step 2 and replace the outer door. Turn the main gas line on and turn the thermostat to the “ON” position.
Bleed any air from the gas line. Hold the pilot button down on top of the thermostat while lighting the pilot. Continue to hold the pilot button down for 20 seconds, then slowly release the button. Repeat this step if the pilot light continues to go out.
Verify you have enough combustion air to keep the pilot light lit. Close all your doors and windows and make sure any other vented gas appliances are running. Hold a lit match next to the water heater vent hood at the top of the water heater.
Watch the match flame. If the flame goes out, the vent may be plugged. It needs to be cleaned of any obstructions. A clogged vent will not allow enough combustion air into the area to feed the pilot light flame, keeping it lit.