Turn thermostat on too heat, it starts fine. When temp is reach, gas goes off, but fan still runs. Open the panel and swicth pops out fan goes off . Round thremostat dial.

Thanks
Jim

Need more info,

Ac? Fan switch turned on? Make model type

Does the fan restart when you replace the panel? If so, likely the fan relay is sticking. Unfortunately that usually means replacing the control board unless it is an older furnace.