Lennox Furnace Troubleshooting

Modern gas furnaces are quite reliable appliances, but even they may be prone to some malfunctions ore breakages caused by misuse or improper maintenance. If you found out that you can’t start your Lennox gas furnace or it shows some signs of a malfunction (provides not enough heat, for instance), you might want to use our troubleshooting tips to save money on professional repair services.

Stage 1
Let’s begin with simple things. Make sure the built-in heater thermostat is on and works fine. Check the batteries and replace them if required. Set the thermostat to “heat” position and adjust its temperature to a higher value that the one you have in the room.
Stage 2
Check the vents in your house – all of them need to be open. Free them from any obstacles and furniture to let the air flow easily. Furnace overheating and other issues are often caused precisely by a poor airflow in the house.
Stage 3
Another cause of a poor operation may have to do with air filters. Once they get too dirty, the hot air cannot properly get through it and you don’t feel enough heat coming out of the appliance. Make sure you replace the filters at least once a year or clean them every few months.
Stage 4
Сlogged gas burners is not a rare problem as well. Carbon deposits on their walls may prevent the furnace from producing enough heat. Remove the burners, carefully clean them and then put them back. Don’t forget: the gas must be turned off.
Stage 5
Inspect the pilot light and try to relight it if it’s out by carefully cleaning it. If you cleaned the pilot light but nothing happened, the cause may hide in poor gas flow which is quite a different story. In that case, you will not be able to fix your furnace without professional help.

Lennox ELO183DH Series Furnace Troubleshooting

ActionSystem Response
Thermostat calls for heat. (W terminal is energized.)Oil primary control is energized.
Ignition system and oil primary control start the furnace. Oil flows as long as cad cell senses flame.
Burner motor is energized and heat fan on delay timing begins. When timing is complete, the circulating fan is energized at heat speed and warm air is delivered to the controlled space.
Thermostat ends call for heat. (W terminal is de-energized.)Oil primary control is de-energized, terminating the burner cycle.
Heat fan off delay (60 to 120 seconds) timing begins. When timing is complete, the circulat- ing fan is de-energized.
Oil primary control and circulating fan are off.
Burner fails to light.Oil primary control locks out within lockout timing (timing depends on oil primary control). Burner motor is de-energized.
If heat fan has started, it continues through the delay off period.
Established flame fails.Burner motor is de-energized and oil primary control goes into recycle mode.
If heat fan off delay is longer than the recycle delay timing, the heat fan continues to run through the next trial for ignition.
Thermostat begins call for cool. (G and Y terminals are energized.)Circulating fan is energized at the cool speed. Cooling compressor turns on immediately.
Thermostat ends call for cool. (G and Y terminals are de-energized.)Circulating fan and cooling compressor turn off immediately.
Thermostat begins call for fan. (G terminal is energized.)Circulating fan is energized immediately at cool speed.
Thermostat ends call for fan. (G terminal is de-energized.)Circulating fan is de-energized.
Limit switch string opens.Oil primary control shut off the burner.
Circulating fan is energized immediately at heat speed.
Oil primary control is de-energized.
Circulating fan runs as long as limit string stays open.
If there is a call for cooling or fan, the circulating fan switches from heat speed to cool speed.
Limit switch string closes.Heat fan off delay (60 to 120 seconds) begins.
Circulating fan turns off after the selected heat fan off delay timing. Oil primary control is energized, initiating burner light off.
Continuous circulating fan is connected. (Optional connectors are available for separate circulating fan speed tap.)Circulating fan is energized at low speed when there is no call for heat, cool or fan.
Humidity control is connected. (Optional connectors are available for 120 Vac humidifier.)Humidifier connections are energized when the burner motor is energized.

Lennox A80UH1E Furnace Troubleshooting

RED LED Flash CodeDiagnostic Codes / Status of Furnace
OffNo power to control or board fault detected
HeartbeatControl powered - displayed during all modes of operation if no errors are detected
1 FlashReverse Line Voltage Polarity
2 FlashesImproper Earth Ground
3 FlashesBurner failed to light, or lost flame during heat demand
4 FlashesLow Flame Signal - check flame sensor
5 FlashesWatchguard - burner failed to light, exceeded maximum number of retries or recycles.
6 FlashesNot Used
7 FlashesPrimary or Secondary Limit Open or Watchguard Mode - Limit Switch Open Longer than 3 minutes
8 FlashesRollout Switch Open
9 FlashesPressure Switch failed to close or opened during heat demand
10 FlashesWatchguard - Pressure Switch opened 5 times during one heat demand
11 FlashesPressure switch stuck closed prior to activation of combustion air inducer
12 FlashesFlame Sensed without gas valve energized
13 FlashesLow Line Voltage

Lennox O23 Furnace Troubleshooting

SourceProcedureCausesCorrection
ThermostatCheck thermostat settings.Thermostat in OFF or COOLSwitch to HEAT.
Thermostat is set too lowTurn thermostat to higher temperature.
Safety OverloadsCheck burner motor, primary safety control, & auxiliary limit switch.Burner motor overload trippedPush reset button pump motor.
Primary control tripped on safetyReset primary control.
Auxiliary limit switch tripped on safetyReset auxiliary limit.
PowerCheck furnace disconnect switch & main disconnect.Open switchClose switch.
Blown fuse or tripped circuit breakerReplace fuse or reset circuit breaker.
ThermostatTouch jumper wire across thermostat terminals on primary control. If burner starts, then fault is in the thermostat circuit.Broken or loose thermostat wiresRepair or replace wires.
Loose thermostat screw connectionTighten connection.
Dirty thermostat contactsClean contacts.
Thermostat not levelLevel thermostat.
Faulty thermostatReplace thermostat.
CAD CellDisconnect the flame detector wires at the primary control. If the burner starts, fault is in the detector circuit.Flame detector leads are shortedSeparate leads.
Flame detector exposed to lightSeal off false source of light.
short circuit in the flame detectorReplace detector.
Primary ControlPlace trouble light between the black and white leads. No light indicates that no power is going to the control.Primary or auxiliary control switch is openCheck adjustment. Set the maximum setting.
Jumper terminals; if burner starts, switch is faulty, replace control.
Open circuit between disconnect switch and limit controlTrace wiring and repair or re- place it.
Low line voltage or power failureCall the power company.
Place trouble light between the orange and white leads. No light indicates faulty control.Defective internal control circuitReplace the control.
BurnerPlace the trouble light between the black and white leads to the burner motor. No light indicates that no power is getting to the motor.Blown fuseReplace the fuse.
Place trouble light between the black and white leads to the blower motor. Light indicates power to the motor and burner fault.Binding burner blower wheelTurn off power and rotate the blower wheel by hand. If seized, free the wheel or replace the fuel pump.
Sized fuel pump
Defective burner motorReplace the motor.

Lennox G1N80BU Furnace Troubleshooting

LED StatusFault Description
LED OffNo power to control or control hardware fault detected
LED OnNormal operation
1 FlashFlame present with gas valve off
2 FlashesPressure switch closed with inducer off
3 FlashesPressure switch open with inducer on
4 FlashesHigh limit switch open
5 FlashesRollout switch open
6 FlashesPressure switch cycle lockout
7 FlashesLockout due to no ignition
8 FlashesLockout due to too many flame dropouts
9 FlashesIncorrect line voltage phasing

Lennox OHR23Q5 Furnace Troubleshooting

Symptoms of Abnormal OperationFault Description(s)Possible Causes
- Motor rocks slightly when starting.- This is normal start-up for variable speed motor.
- Motor won't start.- No movement.- Manual disconnect switch off or door switch open.
- Blown fuse or circuit breaker.
- 24 Vac wires miswired.
- Unseated pins in wiring harness connectors.
- Bad motor/control module.
- Moisture present in motor or control module.
- Motor rocks, but won't start.- Loose motor mount.
- Blower wheel not tight on motor shaft.
- Bad motor/control module.
- Motor oscillates up & down while being tested off of blower.- It is normal for motor to oscillate with no load on shaft.
- Motor starts, but runs erratically.- Varies up and down or intermittent.- Variation in 115 Vac to motor.
- Unseated pins in wiring harness connectors.
- Erratic CFM command from "BK" terminal.
- Improper thermostat connection or setting.
- Moisture present in motor/control module.
- "Hunts" or "puffs" at high CFM (speed).- Incorrect or dirty filter(s).
- Incorrect supply or return ductwork.
- Incorrect blower speed setting.
- Motor starts, but runs erratically.- Stays at low CFM despite system call for cool or heat CFM.- 24 Vac wires miswired or loose.
- "R" missing/not connected at motor.
- Fan is delay mode.
- Stays at high CFM.- "R" missing/not connected at motor.
- Fan is delay mode.
- Blower won’t shut off.- Current leakage from controls into G, V, or W.
- Air noise.- High static creating high blower speed.
- Incorrect supply or return ductwork.
- Incorrect or dirty filter(s).
- Incorrect blower speed setting.
- Excessive noise.- Noisy blower or cabinet.- Loose blower housing, panels, etc.
- High static creating high blower speed.
- Air leaks in ductwork, cabinets, or panels.
- "Hunts" or "puffs' at high CFM (speed).- High static creating high blower speed.
- Incorrect or dirty filter(s).
- Incorrect supply or return ductwork.
- Incorrect blower speed setting.
- Evidence of Moisture.- Motor failure or malfunction has occurred and moisture is present.- Moisture in motor/control module.