What is the MOST LIKELY cause of a drop in natural gas pressure from the main line to the gas manifold?

A drop in natural gas pressure from the main line to the gas manifold is most likely due to a defective pressure regulator at the gas valve. The pressure regulator's primary function is to reduce the high pressure of gas from the main supply line to a safe and usable level for the appliances connected downstream. If the regulator is faulty, it may fail to maintain the appropriate pressure, leading to a significant drop that would affect the operation of appliances and systems relying on that gas.

While other factors can contribute to pressure issues, such as a blocked gas line or insufficient gas supply, these typically manifest in different symptoms or have more complex causes. A blocked gas line would generally result in total loss of flow rather than a gradual drop in pressure. Similarly, an insufficient gas supply would indicate problems further up the line or with upstream pressure, rather than an issue localized to the regulator itself. A malfunctioning pilot assembly may affect burner ignition but would not directly cause a drop in pressure as observed in the gas manifold. Thus, focusing on the pressure regulator as the source of the problem provides the clearest explanation for this situation.