Same as everything else check if valve(s) energized/opens up, smell/sounds of gas, igniter glows, ignition, flame detected, ..... Gas was shut-off, may be..

Goodman Furnace.

https://www.newcentralairconditioner.com/

First Things First – Check Your Gas Supply

Troubleshooting a Gas Furnace When It Fails to Ignite One of the first steps in determining why your furnace is not igniting is to make sure it’s getting gas to it.

Natural Gas Users

If you are on a city gas line you will not be able to do this step because trying to tell if you are getting natural gas to your heater is dangerous and should only be done by a professional HVAC technician. If you smell gas near your furnace when you turn it on there is a good chance that your gas supply is not the problem

Propane or LP Gas Users

It’s much easier to determine if your furnace is getting gas if you are a liquid propane user. First of all, you need to go out into your yard and locate where your gas storage tank is. In the top center of it, there will be a domed area where its fill valve is. There will also be a gas pressure gauge.

Check Your Air Filters

You would be surprised how many gas furnace users do not do this step when their heater does not start up and end up getting charged for an expensive HVAC service call. A gas furnace burns a mixture of air and natural or LP gas. If the air in your system is not flowing it cannot produce the required gas/air mixture and your home heating system will not ignite because of it.

Often times the reason air does not get to your furnace’s igniter is because it is restricted. The main reason for this air restriction is usually traced back to dirty air filters. So check them if your gas furnace fails to ignite when you turn the system on. It’s also a good idea to replace your furnace’s air filters at least once a year.

3. Ignition Source

There are two ways in which a furnace ignites the gas air mixture that comes into it. The oldest way is by a pilot light and over the last 30 years or so these have been replaced by electronic ignition systems. If these fail to do their job your furnace will not come on.

Pilot Light

This is a small flame which is constantly burning inside your furnace. It’s so small it does not consume much gas. If you look at the bottom of your furnace you will see a tray where the burners are. Your pilot light is located to one side of this tray just slightly above it usually.

Look under your furnace to see if it’s lit. If not, you can use a long match to try and carefully light it again. If that does not work then you have a bigger problem.

Electronic Furnace Ignition

With an electronic furnace igniter switch, your furnace is safer and does not use the small amount of gas that a furnace with a pilot light does. These igniters are located in about the same position as where we described the pilot light as being. They are made of a very lightweight metal alloy that gets super-hot when an electric current goes through it and this ignites the gas/air mixture coming into your furnace.

Look under your furnace and see if your igniter glows when your furnace tries to start up. You will also hear a clicking noise that is a sign your heater system is trying to come on. Repeated clicking in small intervals usually means your igniter is not coming on and working.

Thermostat

If you turn your heater on and then move the thermostat to a higher setting several things should happen that will make the furnace ignite. The first sounds you will hear are that of the system’s fan which brings air into it to start the burning process. After that, you will hear the swoosh sound of the gas igniting. After a few seconds, hot air will then flow out of the vents in the rooms of your home.

None of this will happen of course if your thermostat is not working properly. What’s the easiest way to tell you to have a defective thermostat? None of the sounds we just described will take place because your broken thermostat did not communicate with your furnace that it’s time to come on. All will be silent near your gas furnace unit.

Fortunately, the thermostat on your gas furnace controls is pretty easy to replace yourself. Just open your control box and locate it. Take it out and then go to your local HVAC supplier and have them match it up with a new thermostat. To install it just reverse the process you used to take it out.