Problem: Gas furnace will not ignite the gas to produce heat for our home. What could be the problem?
Answer: Your furnace probably has a bad ignitor. The following sequence of operation is what I see most of the time when an ignitor is bad.
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. Please click here if you are interested in seeing the furnace hot surface ignitors we sell. I would suggest that you inspect your ignitor closely for cracks. Make sure you do not touch the ignitor (grey glass black part) with your bare hands. You can handle the ignitor by the white porcelain base of the ignitor. If you do not visibly see a crack then you can test the ignitor with an ohm meter to see if it is good or bad. Set the multimeter so it can properly measure a resistance of 10 to 200 ohms. Disconnect the hot surface ignitor from the control board and measure the resistance. A good hot surface ignitor will have a resistance of 40 to 90 ohms. Greater than 90 ohms indicates a failing or failed hot surface ignitor. Please see the video below which shows how to test and ignitor with an ohm meter. If the ignitor looks and tests to be good then you could have a furnace control board problem or a limit, rollout switch or pressure switch problem. The furnace’s control board might not be supplying the voltage to the ignitor. Please click here for our Furnace Control Boards Page. If your furnace lights and the gas stays on for 8 to 10 seconds, then shuts right back off, then I would suggest that you clean your flame sensor with light sand paper (emery cloth) 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. We sell flame sensors on the following page: Please click here if you are interested in seeing the furnace flame sensors we sell. We sell hot surface ignitors on the following page: Please click here if you are interested in seeing the furnace hot surface ignitors we sell. We sell 56 different ignitors. Please send your furnace make and model number if you would like for us to look up the ignitor or other parts for your furnace. If you have a Bryant or Carrier furnace we will need the product number in order to look up parts. If you have a Rheem or Ruud we will need both model number and serial number. Our email address is: firstname.lastname@example.org You can also leave a comment in the comments section below and we will respond to your comment ASAP. Below we have two real good informative Youtube videos produced by Fix It Home Improvement Channel and Repair Clinic on how to test, replace a furnace ignitor and flame sensor. Thanks to Fix It Home Improvement Channel and Repair Clinic for making these informative videos.