How do I know if my hot surface ignitor is bad? How do you test a hot surface ignitor?

As a hot surface igniter is a resistance (thermal resistance producing heat), the only way to check if an ignitor is bad or broken is to check the resistance value.

You will need to use a ohmeter or multimeter to measure the cold resistance (when off) value of the igniter. Set the multimeter so it can properly measure a resistance of 10 to 200 ohms (at room temprature 21~23°C). Disconnect the hot surface ignitor from the control board and measure the resistance at the two electrodes (no polarity). A good silicon nitride hot surface ignitor will have a resistance of 30 to 75 ohms. Greater than 75 ohms indicates a failing or failed hot surface ignitor. If you get 0 or ∞ or no reading at all, it mean that the resistance is broken, so the igniter is broken and should be replaced.