Error lights on Lennox G23x.


Old 10-31-09, 05:33 AM
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Join Date: Oct 2005
Location: Missouri
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Error lights on Lennox G23x.

I have Lennox G23x with Surelight control. The furnace would not come on at all, and the diagnostic lights displayed the following: Left: slow flash; right: on. According to the chart explaining the codes, that display means, "primary/secondary limit switch open."

I switched off the main power, then turned it back on, and the furnace started, first the combustion air blower came on, then the burners ignited. The main blower then came on. The green diagnostic lights displayed the following: left: slow flash; right: fast flash. That apparently means, "Low flame signal, check flame sensor."

The furnace operated for a few minutes, then the burner turned off, but the blower continued to operate. The furnace later shut off before the temperature rose enough to trip the thermostat in the living room.

Any idea what's happening and how to fix it? Thanks for your help.
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Old 10-31-09, 07:37 AM
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The most likely problem is that the flame sensor is covered with oxides which prevent the ignition control from reliably detetcing that the main burners have lit properly.

The flame sensor is on the opposite side of the furnace from the hot surface igniter. It a small rod about 1/8" in diameter sticking up above the burner with a wire coming from the bottom going back to the circuit board.

Take that out and clean it with a wire brush and reinstall it.

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