Gas furnaces depend upon an ignition system to ignite the fuel that will create the heat. Igniters can fail, though, which means you will need to have the ignition replaced if you want to heat your home.
1. Poor Heating
Poor heating can be a result of a variety of issues, but it is still one of the first symptoms of an ignition problem. When the ignition fails, the furnace may still run but there will be no hot air so only cold air will come from the vents. Verify that the thermostat is set to the desired temperature and on the "furnace" setting. If it is and poor heating is still a problem, then it's time to have the ignition inspected to see if it is the problem.
2. Frequent Cycling
A cycling furnace is one that is coming on and then turning off a few minutes later, just to come back on again and repeat the entire process. Not only is it annoying, but a cycling furnace will also suffer from greater wear and tear while heating the house poorly. Cycling can be caused by ignition failure. The furnace attempts to come on, but the igniter doesn't ignite the fuel so the furnace cycles down a few minutes later. Frequent cycling due to ignition problems usually happens in conjunction with poor heating.
3. Abrupt Shutdowns
Most furnaces have safety switches that shut down the system in the event of ignition and fuel combustion issues. Unlike cycling, your furnace will power on and then immediately go into shutdown mode without trying to cycle back on again. The reason for the shutdown is because the fuel isn't igniting and burning off at all, so the safety sensor shuts down the furnace so that the fuel won't be leaking into your home without igniting.
4. No Flame
Sometimes it's easy to see if there is an ignition problem. You can open the door to the combustion chamber and check for the pilot light, which should be a small unwavering blue flame. If there is no flame when the furnace clicks on, or if the flame is inconsistent or any color but blue, there is an issue in the ignition system or fuel valve. Some furnaces have a flameless ignition system, which uses a heated element to ignite the fuel. You should be able to see the glow from the element at the time of ignition if it is working properly.
Contact a furnace repair service if you suspect there are issues affecting your ignition system.