Lennox furnace error codes

Lennox furnaces feature control boards with two LED lights, which blink in certain patterns, depending on the time of issue occurring. Signals are coded in such a way that users can memorize the codes’ meanings.  This helps them identify issues, as the codes tell the owners where to look for the problem.

Furnace Not Working

There are codes, which appear when a Lennox MCF80 fails to work in the cooling, heating or continuous fan modes. If none of the lights is on, this may be due to poor wiring, electricity shutoff, tripped circuit breaker, faulty control board, door lock or transformer. When there is a roll-out switch problem, Lighht 1 will burn steadily, and Light 2 will be blinking slowly. The roll-out switch is intended for detecting flames reaching a place or item, which should not be exposed to them.

The combination with Light 1 blinking fast and Light 2 blinking slowly indicates a wrong polarity. Both lights blinking slowly signal a motor failure or a problem in the motor circuit. Both lights blinking alternatively and quickly can indicate three issues:

  • Voltage in the power line dropping below 75 volts;
  • The six-pin connector in the circuit board is misplaced and not connected properly;
  • The ground  is bad.

Air Blower Functioning Only

Both lights blinking fast and alternatively mean that the furnace has stopped working except for the air blower. Most likely, the ignition is bad, or its circuit is open. It is advisable to check the ignitor’s wiring: it may be loose, damages, or not properly connected. Please, check continuity across the ignitor using a multimeter. It should not drop below 10.9 ohms or exceed 19.7 ohms.

Air Blower/Heater Not Functioning

When Light 2 is on, and Light 1 is blinking slowly, there is a problem in the limit switch, as it is not letting the air blower and heater work properly. However, the cooling system stays functional. Normally, the limit switch is activated when there is a likelihood of overheating. Most likely, the switch’s wiring is poor or disconnected. Some furnaces have two limit switches: a primary and a secondary one.

A combination of Light 1 off and Light 2 blinking slowly signals a problem in the air blower: most likely, there is a problem in the wiring, or the pressure switch is not opening. Please, check the air blower itself, because eit could be faulty.

Heat and Ignitor Not Functioning

If the heat more and the ignitor are not working, and the air blower is, you are likely to see Light 1 off and Light 2 flash slowly. This can be due to a stuck pressure switch. Not unlikely, its tubing has gotten blocked. Also, there may be a problem in the wiring or the switch itself. If there is some pressure in the unit, and the switch does react, check the vent: it may be blocked or poorly installed.

Ignitor and Air Blower are Functioning, No Heat

If the air blower and ignitor are working, but there is no heat, the two lights will show slow alternate blinking. This may be due to a gas supply problem, faulty gas valve or poor ignitor control.

Furnace Stops Too Early

Light 1 off and Light 2 blinking slowly indicate that the heat mode has stopped prematurely. This could be due to unstable pressure in the unit. Not unlikely, the slame sensor or sensor wire has been damaged or it is poorly connected. Also, it may indicate the inability of the control unit to recognize flame.

Light 1 burning steadily with Light 2 flashing slowly indicate a possible gas leak, heat exchanger blockage or incorrect burner position. Also, it may be a signal of the furnace’s current heat output exceeding the admissible level. If the lights are blinking slowly and alternately, there imay be a crossover problem with the gases recirculating inside the furnace.  Finally, there may be a wiring problem bear the sensor.

Low Flame Code

If there are no problems with the furnace’s general functioning, but there is the low flame code (Light 1 flashing slowly with Light 2 flashinng quickly). This may be due to a dirty or improperly positioned flame sense rod.