A bad thermocouple is the most likely culprit when your Whirlpool water heater fails to heat the water in the tank. The pilot light heats the thermocouple, which generates electric current that tells the gas control valve/thermostat to continue to feed gas to the burner assembly. If the pilot light should go out, the thermocouple closes the gas control valve/thermostat by interrupting the electrical current.
Turn off the gas at the manual shut-off valve that supplies the gas to control valve/thermostat by turning it clockwise. Turn off the gas control valve/thermostat by turning the thermostat to the lowest position, and turning the control knob on top of the valve to the "OFF" position. (The gas control valve/thermostat is a single part; it controls the flow of the gas through a manifold tube as you adjust the thermostat.)
Remove the outer door from the bottom of the water heater by grasping the door with your hands, and pulling the metal tabs attached to the outer door from the slots on the body of the water heater. The outer door covers the manifold door assembly, thermocouple, manifold tube and pilot tube, protecting them from damage.
Remove the two screws that secure the manifold door assembly to the tank. Use an adjustable wrench to loosen the thermocouple from beneath the gas control valve/thermostat by turning it counterclockwise. The thermocouple is a thin copper tube attached to the left of the manifold tube. Use the adjustable wrench to loosen the pilot tube by turning it counterclockwise. The pilot tube is a small-diameter tube located to the right of the manifold tube.
Loosen the manifold tube from the gas control valve/thermostat by turning it clockwise with the adjustable wrench. The manifold tube is a flexible, corrugated metal tube that attaches to the gas control valve/thermostat between the pilot tube and the thermocouple. It is the largest of the three tubes.
Pull the wire from the pilot igniter button connected to the gas control valve/thermostat, and pull the two manifold burner assembly connector wires from the gas control valve/thermostat. Grasp the igniter wire and the two manifold burner assembly connector wires in your hand and pull down on them, pulling them out of the valve. Grasp the manifold tube in your hand and pull down and away from the gas control valve/thermostat, releasing the thermocouple, pilot tube and manifold tube from the valve.
Remove the manifold burner assembly from the burner chamber by pulling it out and away from the water heater. Remove the two piece wire connector that secures the thermocouple and igniter wire through the manifold door by removing the retainer clip holding it.
Pull the thermocouple from the pilot bracket attached to the manifold burner assembly. Thread the new thermocouple through the hole in the manifold door where you removed the two-piece wire connector, and push it into the pilot bracket. Replace the two-piece wire connector onto the thermocouple and igniter wire, securing it with the retainer clip, and slide the manifold burner assembly back into the water heater.
Reattach the manifold tube, pilot tube and thermocouple to the gas control valve/thermostat and tighten them using the adjustable wrench. Replace the igniter wire into the igniter, and push the two manifold burner assembly connector wires into the gas control valve/thermostat.
Replace the screws in the manifold door assembly that you removed in Step 2, and replace the outer door. Turn on the gas at the manual shut off by turning the valve counterclockwise, and turn on the gas supply valve/thermostat on the water heater by turning the thermostat to the desired setting and the gas control knob to the "ON" position.