Furnace Burners Light Then Shut Off: Cleaning the Flame Sensor
The short version
A furnace that lights its burners for two or three seconds and then shuts off is often failing at the flame detection step of the six-step startup sequence. The video walks through the normal startup, shows the failure, then removes the flame sensor and cleans carbon off the rod with steel wool or a dollar bill, or replaces it.
Full written account
A furnace that lights its burners and drops them after two or three seconds is, in the most common case the presenter describes, failing at one step: flame detection. Two of his last five furnace failures in rental properties were this exact issue. One needed no parts and the other about ten dollars in parts.
The six-step startup
The sequence is: the draft inducer motor starts; a pressure switch confirms the draft; the igniter glows hot; a relay opens the gas valve and the burners light; a flame detection sensor confirms the flame; and after about 30 or 45 seconds the main blower fan starts. On the furnace shown, the full cycle from the thermostat calling for heat to the blower takes about a minute and 20 seconds.
The failure
In the failure mode the process reaches step four. The burners start, then stop after two or three seconds, so steps five and six never happen. After three, four or five cycles the furnace goes into a lockout mode that lasts for hours. This is a safety protocol: the control board will not leave the gas valve open if no flame is confirmed.
Cleaning the sensor
The flame detection sensor passes a small current to the control board to confirm the flame. Carbon buildup on its rod can stop it sensing the flame. To remove it, take the connector off the spade terminal and back out one screw with a quarter inch nut driver. Then work off the carbon with steel wool or a one dollar bill, and reinstall it.
Replacing it
A replacement sensor needs the same shape, straight or 90 degree, and the mounting hole in the same place, because that orients the rod into the path of the flame. On this furnace the rod points toward the last burner. A sensor that puts the rod in the middle of the burners will not work effectively. Packs of three or four cost about 15 dollars online, and he suggests keeping spares for rental properties.