What safety device is commonly found in combustion appliances to prevent accidents?

A flame sensor is a critical safety device commonly found in combustion appliances. Its primary role is to detect the presence of a flame during operation. If the flame goes out or if the sensor detects any issues with the flame that could indicate unsafe combustion, the flame sensor will shut off the gas supply to prevent the buildup of unburned gas, which could lead to dangerous conditions such as explosions or gas leaks. This immediate response enhances safety by ensuring that the combustion process remains stable and that the appliance operates within safe parameters.

Other devices listed may play various roles in the functioning of combustion appliances, but they do not focus directly on preventing dangerous situations caused by flame failure. For instance, a thermostat override typically serves to control temperature settings rather than directly preventing accidents related to flame detection. A pressure gauge measures the pressure in the system, which, while important, does not actively prevent unsafe events. A check valve helps to prevent backflow in gas lines but does not monitor flame presence. Therefore, the flame sensor stands out as the device specifically designed to enhance safety by intervening in the event of a combustion failure.