Which of the following is a sign that a gas appliance may need service?

A sign that a gas appliance may need service is indicated by yellow flames and soot buildup. When gas appliances are functioning correctly, they should produce a steady, blue flame, which indicates proper combustion. A yellow flame suggests that the appliance is not combusting gas efficiently, potentially due to inadequate oxygen supply or a malfunction. This can lead to incomplete combustion, causing soot buildup which can not only impair the appliance's performance but also pose safety risks such as the release of carbon monoxide.

In contrast, consistent blue flames typically reflect proper operation of the appliance, while a decrease in energy bills could suggest improved efficiency, possibly due to better usage or upgrades in technology rather than a need for service. Lastly, no noticeable smell or sound often indicates that an appliance is functioning normally, as gas appliances usually operate quietly and without odor under ideal conditions. Thus, the presence of yellow flames and soot is a clear signal that immediate attention is necessary for safe and efficient appliance operation.