A Lennox 80MGF series furnace has an internal control board with a LED diagnostic light feature. The units have two LED lights that provide troubleshooting codes in the form of flash patterns. If you know what the light codes mean, you can use the codes to narrow down the potential reasons why the furnace is not operating properly.
Furnace Won't Operate in Any Mode
Some codes are seen if the Lennox MCF80 won't work in the heating, cooing or continuous fan mode. If both LED lights are off, there might be a problem with the furnace wiring, supplied voltage, circuit breaker, transformer, control board or door lock. When LED No. 1 is steady, but LED No. 2 flashes slowly, it's signaling a problem with the roll-out switch. The roll-out switch is a safety device designed to register any flames that are reaching a spot in the furnace fire shouldn't touch.
If LED No. 1 flashes fast, but LED No. 2 flashes slowly, the main power polarity is reversed. If both lights flash slowly, the blower motor has failed or its circuit is open. Alternating quick flashes on both LED lights usually means one of three problems: The furnace isn't properly grounded; the six-pin connector wasn't attached to the circuit board correctly; or the line's voltage has gone below 75 volts.
Only Air Blower Works
A specific LED code applies if the Lennox furnace doesn't heat or cool but the air blower works continuously. Fast flashes that alternate on both LED lights means the ignitor has failed or has an open circuit. The ignitor circuit might have loose or bad wires, or the wiring wasn't connected correctly. A resistance meter can be used to read the resistance across the ignitor, which should fall between 10.9 and 19.7 ohms.
Air Blower and Heat Mode Not Working
If the air blower isn't starting and the furnace won't respond to calls for heat but does cool, a particular set of LED codes apply. If LED No. 2 is on but LED No. 1 is flashing slowly, the limit switch might be incorrectly wired or open. A limit switch prevents the furnace from overheating, and some 80MCF furnaces have primary and secondary limit switches.
If LED No. 1 is dark but LED No. 2 has a slow flash, the furnace or air blower might have been wired or connected incorrectly, or the pressure switch is stuck closed. In some cases, this code means the air blower is dead.
Heat and Ignitor Won't Start
If the heat mode won't kick on and the ignitor sequence won't start but the air blower starts, LED No. 1 might go dark while LED No. 2 slowly flashes. This indicates a problem with the pressure switch or the tubing that connects it to the furnace. The tubing might be blocked or damaged or the switch itself has failed or wasn't wired correctly. If the switch is reacting to a problem with the level of pressure in the unit, a vent might be blocked or not properly installed.
No Heat But Ignitor and Air Blower Work
If the furnace won't heat but the air blower and ignitor are starting, both LCD lights might alternate a slow flash. This code indicates a problem with the gas supply to the furnace, or a problem with the gas valve or ignitor control.
Furnace Shuts Down Early
If the furnace's burners light but the heat cuts off before the home reaches the temperature called for by the thermostat, LED No. 1 might turn off while LED No. 2 flashes slowly. The code indicates a problem with the pressure inside the unit. If both lights alternate and flash slowly, the furnace may have a damaged or disconnected flame sensor or sensor wire. Occasionally, this code means the unit's control doesn't recognize the burner's flame.
When LED No. 1 is on but LED No. 2 is a slow flash, there might be a gas leak at the manifold, a blocked heat exchanger or a problem with the burner position inside the furnace. This code may also mean the furnace is exceeding its maximum heat output. If both lights have an alternating slow flash, the burner's crossover might be restricted, the gases inside the furnace are recirculating or the sensor wire is loose.
Low Flame Code Only
If the furnace is working correctly but the lights display the low flame code -- LED No. 1 flashes slowly while LED No. 2 flashes quickly -- the flame sense rod might be dirty or on the wrong spot on the burner.
Anna Assad began writing professionally in 1999 and has published several legal articles for various websites. She has an extensive real estate and criminal legal background. She also tutored in English for nearly eight years, attended Buffalo State College for paralegal studies and accounting, and minored in English literature, receiving a Bachelor of Arts.