A thermocouple is a device within a gas-powered water heater that operated to automatically turn the heater's supply of gas off in the event that the pilot light extinguishes. Thermocouples need periodic replacement, as the tip of the thermocouple is subject to corrosion. A pilot flame which is predominantly blue in colour is symptomatic of a failing thermocouple. So too is the presence of ash on the thermocouple's tip. Thankfully, installing a universal thermocouple is a straightforward task which only takes a few minutes.
Turn the water heater's supply of gas off by twisting the knob on top of the control panel to the "Off" position. Allow at least ten minutes to pass to allow the gas fumes to dispel.
Disconnect the thermocouple from the bottom of the control box by turning the nut on end of the thermocouple in a counterclockwise direction with a wrench. There are three tubes connected to the bottom of the control box. The thermocouple is the smallest of the three tubes.
Remove the access panel on the bottom of the water heater. The access panel can usually be removed by simply pulling it away from the water heater, although some access panels are held to the water heater with one or more screws which must first be removed with a screwdriver.
Trace the thermocouple into the interior of the water heater until the end of the thermocouple is reached. Note that a nut holds this end of the thermocouple in place.
Remove the thermocouple from the interior of the water heater by turning the nut on the end of the thermocouple in a counterclockwise direction with a wrench until the thermocouple is loose, and then pulling the thermocouple out of the water heater.
Install the replacement thermocouple into the interior of the water heater, and then tighten the nut with a wrench to lock the thermocouple into place.
Attach the opposite end of the thermocouple to the control box by turning the nut on the thermocouple in a clockwise direction with a wrench.
Twist the knob on top of the control box to the "Pilot" setting.
Light the pilot light next to the pilot gas tube with a barbecue lighter or a long match, and then press and hold down the reset button on top of the control box for one minute after the pilot is lit.
Twist the control knob to the "On" position.
Reinstall the access panel to complete the installation.