The Hot surface ignitor comes on and lights the burner, but goes off after about 9 seconds. It repeats this and then locks out. What could be causing this.

Intertherm Furnace: Model #MMHB-56ABHR-01

I’m not familiar with the Intertherm, but most furnaces have a flame detection probe that the flame of one of the burners will contact. These sometimes will get an oxide buildup on them causing them not to respond correctly. Using fine sand paper to remove the buildup will most of the time restore normal operation. In extreme cases, the probe needs to be replaced.

Eric D