If your furnace is not heating sufficiently then it obviously means that something is wrong with the unit. Sometimes the problem will be not that complicated and can hence be fixed on your own. To fix easy problems you need to figure out the issue by inspecting your furnace. Check the thermostat, power, gas valve, filter, ignition, flame sensor and pressure switch and if there is problem with any of these components then it need to be fixed to restore your furnace to its working condition. Fixing a furnace on your own is fine only if the problem is not complicated hence you need to understand when a technician needs to be called. If the fix requires opening the furnace and replacing the parts inside the unit then you need to seek the help of a professional.
Most of the time the problem will be with the thermostat check the thermostat to find out if there is a problem. If the thermostat shows a jumbled digital display then it means that it needs to be replaced. If the battery indicator is flashing then it means that the batteries have to be replaced. Sometimes there won’t be any problem with the thermostat it is just that you haven’t set the right temperature. Set the right temperature and check whether your furnace is working. Also if the function is not set rightly then the furnace won’t be heating. Another aspect to check is the fuse of the furnace. If it is blown then replacing it with a new one will solve the problem.