Lennox furnace troubleshooting

Lennox manufactures home and commercial heating and cooling systems. Established in 1895 in Marshalltown, Iowa, it is now an intercontinental company. There are several product lines allowing you to choose HVAC equipment that will best suit your needs.
Is your Lennox furnace not working? If the appliance doesn’t heat or works poorly, the tables below will be of great help. They provide expert advice on furnace repair and adjustment. It will allow you to troubleshoot and fix typical problems. You will probably want to try the simplest ones first. In some cases, troubleshooting your furnace will only require pressing a button.

ActionSystem Response
Thermostat calls for heat. (W terminal is energized.)ST9103A closes oil primary control T-T connections. Ignition system and oil primary control start the furnace. Oil flows as long as oil primary control 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 timing begins. When timing is complete, the drculating fan is de-energized. ST9103A returns to standby mode (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 selected delay off period.
Established flame fails.Burner motor is de-energized and oil primary control goes into recycle mode. If selected 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. ST9103A may be fadory-configured to operate heat speed in this mode.
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. ST9103A opens oil primary control T-T connections. 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 doses.ST9103A begins heat fan off delay sequence. Circulating fan turns off after the selected heat fan off delay timing. ST9103A closes oil primary control T-T connections. 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. If fan operation is required by a call for heat. cool, or fan. the ST9103A switches off the continuous fan speed tap before energizing the other fan speed.
Electronic air deaner is connected. (Optional connectors are available for 120 Vac electronic air deaner.)Electronic air deaner (EAC) connections are energized when the heat or cool speed of the circulating fan is energized. EAC connections are not energized when the optional continuous fan terminal is energized.
Humidity control is connected. (Optional connectors are available for 120 Vac humidifier.)Humidifier connections are energized when the burner motor is energized.

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 circuitBroken 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 replace 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.
Oil SupplyCheck tank gauge or use dip stick.No oil in tankFill tank.
Coat dip stick with litmus paste and insert into bottom of tank.Water in oil tankIf water depth exceeds 1 inch, pump or drain water.
Listen for pump whine.Tank shut-off valve closedOpen valve.
Oil Filters & Oil LineListen for pump whine.Oil line filter is pluggedReplace filter cartridges.
Kinks or restriction in oil lineRepair or replace oil line.
Plugged fuel pump strainerClean strainer or replace pump.
Open bleed valve or gauge port. Start the burner. No oil or milky oil indicates loss or prime.Air leak in oil supply lineLocate and correct leak.
Tighten all connections.
Oil PumpInstall pressure gauge on pump and read pressure. Should not be less than 140 psi.Pump is partially or completely frozen. No pressure and the motor locks out on overload.Replace pump.
Coupling disengaged or broken no pressureRe-engage or replace coupling.
Fuel pressure too lowAdjust to 100 psi.
NozzleDisconnect ignition leads. Observe the oil spray (gun assembly must be removed from unit). Inspect the nozzle for plugged orifice or carbon build-up around orifice.Nozzle orifice pluggedReplace nozzle with the same size, spray angle, and spray type.
Nozzle strainer plugged
Poor or off center spray
Ignition ElectrodesRemove gun assembly and inspect electrodes and leads.Fouled or shorted electrodesClean electrode leads.
Dirty electrodes and leads
Eroded electrode tipsClean electrode tips and reset the gap to 5/32 inches and correctly position tips.
Improper electrode gap spacing
Improper position of electrode tips
Bad buss bar connectionRetension and realign.
Cracked or chipped insulatorsReplace electrode.
Cracked or burned lead insulatorsReplace electrode leads.
Ignition TransformerConnect ignition leads to the transformer. Start burner and observe spark. Check line voltage to transformer primary.Low line voltageCheck voltage at power source. Correct cause of voltage drop or call the power company.
Burned out transformer windings.Replace the transformer.
No spark or weak sparkProperly ground the transformer case.
Burner MotorMotor does not come up to speed and trips out on overload. Turn off power and rotate blower wheel by hand to check for binding or excessive drag.Low line voltageCheck voltage at power source. Correct cause of voltage drop or the call power company.
Pump or blower overloading motorCorrect cause of overloading.
Faulty motorReplace motor.