How the board proves there's a fire
Furnaces, water heaters and fridges all light the same way: open the valve, spark, then prove flame within a few seconds or shut the gas off.
The short version
- The flame itself conducts a tiny current from the sense electrode through the fire to the grounded burner. That's what the board is watching.
- Lights, burns a few seconds, then faults: the burner is fine and the board didn't see the flame. That's the sense side, not the gas side.
- An oxidized electrode, a wide gap or a cracked ceramic weakens the signal. Clean it with abrasive, set the gap to spec, don't bend it by the ceramic.
- The ground path matters as much as the electrode. Ohm the burner to the board ground. Rust and loose screws break it.
- Never lights at all is a different problem: check for spark, 12V at the valve during the trial, and gas pressure.
Numbers
- Burner to board ground: under 1 Ω
- Typical trial for ignition: about 6 to 8 seconds, then lockout after 3 tries
- Electrode gap: about 1/8 in (furnace, water heater); 1/8 to 3/16 in (Norcold)
Shop tip
Carry a strip of emery cloth. Half the flame-sense faults you'll see are a dirty electrode.
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.