Your water heater thermocouple is a key element in heating the water in your home. Since there could be problems other than a defective thermocouple for your water failing to heat, you may not know you have a defective thermocouple until you replace the one in your water heater.
How to Identify and Locate Your Water Heater's Thermocouple
To locate your water heater's thermocouple, open your heater's gas burner doors. One end of these wires will be connected to the heater's gas control valve. The other end of the wire will be attached to the pilot light assembly. These wires are your water heater's thermocouple.