My furnace blower shuts off after 20 seconds of running

Most likely one of two issues is occurring. Either the inducer fan is not providing enough draft because it is either spinning too slowly or the impeller is damaged, or the pressure switch that the inducer motor air flow is supposed to activate is not operating properly. If the pressure switch fails to activate, the furnace will not open the gas valve and fire the ignition believing that there is not sufficient draft the vent the combustion gases.

You have a dirty or bad heat sensor. When you remove the front panel on you furnace you will see a probe that is 2 or 3 inches long that is in the flame when the furnace fires up. If it doesn't detect a certain temperature within the first 30 or 40 seconds it shuts down the furnace. You can either clean the probe off with sandpaper, or purchase a new one. They aren't very expensive and are easy to replace. In my experience it is usually a build up of carbon on the sensor and a good cleaning with sandpaper and alcohol usually takes care of the problem.

The first thing I would do is make sure the flame sensor is clean, this is a metal rod immersed in the burner flame on the opposite side of the igniter. If too much oxidation builds up on it, it will not allow the burner to stay lit. To clean, just turn off furnace and remove flame sensor and clean with steel wool and then reinstall. If this does not solve the problem there may be a problem with the gas valve.

On the call for heat from the thermostat the inducer should come on first, a pressure switch proves that the inducer is moving enough air up the stack, closes and energizes the ignition and burners come on. 30 to 60 seconds later the blower comes on. Everything should stay on as long as the thermostat is still calling for heat. When the thermostat is satisfied the burner and inducer shut off and the fan will run to remove the heat from the heat exchanger and then shut off. Cycle complete. Some inducers have a post purge so when the thermostat is satisfied the burner shuts off, the inducer runs for 30 seconds or so and then shuts off. I am thinking that your furnace may be running on the limit switch and when the burner is off but the inducer stays on, maybe the limit is shutting of the inducer. Check your air filter and make sure all your heat registers are open and that the return air grill is not blocked.

Hello, sounds like the limit switch is opening on the furnace.Usually when a furnace goes off on its high limit is becuase the airflow is being restricted making the furnace overheat. a limit switch is there to protect the furnaace from overheating, most cuases are either dirt or the blower motor not running up to speed, anything that cuts down on the airflow will cause the high limt switch to open.

check for power at gas valve.

1. Thermostat calls for heat. 2. Draft inducer motor starts. 3. Pressure switch attached by a small plastic or rubber tube senses the negative pressure produced by the draft inducer and closes. 4. Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound. The ignitor did not glow, the flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the hissing sounds stops with no ignition of gas to heat your home. Your furnace shuts down and goes into a lock out condition until you turn your power switch back off and on again. Then the sequence starts all over again with no ignition of the gas. Solution:You probably need to purchase and install a new ignitor. I would suggest that you inspect your ignitor closely for cracks. Make sure you do not touch the ignitor with your bare hands. If you do not visually see a crack, then you could have a furnace control board problem or a limit, rollout switch problem. The furnace's control board might not be supplying the voltage to the ignitor. If your furnace lights and the gas stays on for 8 to 10 seconds, then shuts right back off, then you need to clean your flame sensor with light sand paper or steel wool. You might need a new flame sensor, but most of the time they can be cleaned an will work well after cleaning.

Problem: Your furnace will not ignite the gas to produce heat for your home. When a furnace has a bad ignitor what I see most of the time is the following sequence of operation:

1. Thermostat calls for heat. 2. Draft inducer motor starts. 3. Pressure switch attached by a small plastic or rubber tube senses the negative pressure produced by the draft inducer and closes. 4. Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound. The ignitor did not glow, the flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the hissing sounds stops with no ignition of gas to heat your home. Your furnace shuts down and goes into a lock out condition until you turn your power switch back off and on again. Then the sequence starts all over again with no ignition of the gas. Solution: You probably need to purchase and install a new ignitor. I would suggest that you inspect your ignitor closely for cracks. Make sure you do not touch the ignitor with your bare hands. If you do not visually see a crack, then you could have a furnace control board problem or a limit, rollout switch problem. The furnace's control board might not be supplying the voltage to the ignitor. If your furnace lights and the gas stays on for 8 to 10 seconds, then shuts right back off, then you need to clean your flame sensor with light sand paper or steel wool. You might need a new flame sensor, but most of the time they can be cleaned an will work well after cleaning.

Check gas pressure, inspect manifold & make sure there is not a blockage.Make sure gas is present and main gas shut off valve to furnace is on also check the knob on the 24 volt gas valve and see if it is in the on position , check the MV terminal on the gas valve to 24 volt common with a volt tester to see if you have 24 volts present, if you do have 24 volts at gas valve, replace valve. if you do not have 24 volts at gas valve check high limit control & roll out limits for a open circuit, Good luck.

it sounds like the afc the fan control not working could be dust on the afc