The proper function of most gas appliances relies, in part, on a thermocouple.
A thermocouple is a safety device intended to avoid the venting of unignited gas from an appliance burner into a room. A functional thermocouple acts as a relay that senses the heat from a pilot light then holds a valve open that controls the main gas flow to the burner. On stoves, thermocouples are most commonly found in the oven cabinet.
How a Thermocouple Works
A thermocouple consists of a bulb that must be located directly in the flame of a pilot light. A chemical inside the thermocouple bulb is heated by the pilot flame and expands; this expansion holds open a valve that allows gas flow to the control knob. When the control knob is switched on, the burner is supplied with gas that is ignited by the pilot flame.
What Happens When a Thermocouple Goes Bad
A cold thermocouple interprets the lack of heat as an extinguished pilot light. Without a pilot light, gas could be vented from the appliance into the room without being ignited and swiftly build up to dangerous levels. The thermocouple prevents this from happening. In basic terms, when a thermocouple goes bad, the appliance should not light. The usual symptom that a thermocouple has gone bad is the absence of gas at the burner -- when the control knob is turned on -- if there is a good flame at the pilot assembly. However, this symptom can be the result of several failures in the system, all of which should be checked before investing in a new thermocouple.
Check the Gas Supply
Pilot lights sometimes get blown out. Following the stove manufacturer's instructions, try to reignite the pilot. If this is not possible, it may be that the problem is due to no gas being available to the appliance. Before tackling any more complex diagnoses, ensure that your tank is not empty if you use propane. Check that the local utility is not experiencing problems if you use main natural gas. Light a different burner on the stove or another appliance, such as a water heater or a gas fire, to confirm you have gas.
Other Factors to Eliminate
If there is gas to the stove but the pilot refuses to light, there are several potential reasons that are indirectly related to the thermocouple but do not indicate it should be replaced. Visually check that the tiny holes in the pilot burner head are not obstructed; in ovens this is often caused by food spills. If the thermocouple bulb has been jarred out of position by accidental contact, it will not work; check that the bulb is directly in the path of the flame. Contamination in the pilot assembly can restrict gas flow, resulting in a too-small flame or a yellow flame. A yellow flame does not burn well -- gas flames should be blue -- and may not be hot enough to trip the thermocouple.
Tags: pilot light, control knob, check that, flame pilot, flame yellow, flame yellow flame
