Lennox fault codes

Likely causes, most common first

1. Hot and neutral wires reversed at the furnace junction box
2. Reversed polarity in the branch circuit or receptacle feeding the furnace

What you can safely check yourself

• Confirm the furnace disconnect switch and the breaker feeding the furnace are on
• Turn the furnace power switch off for 30 seconds and back on once to see whether the code returns

Likely causes, most common first

1. Ground wire missing, loose or corroded at the furnace junction box
2. Furnace chassis not bonded to a proper earth ground

What you can safely check yourself

• Confirm the furnace disconnect switch and breaker are on
• Turn the furnace power switch off for 30 seconds and back on once to see whether the code returns

Likely causes, most common first

1. No gas supply to the furnace, or the manual gas valve is closed
2. Dirty or failed flame sensor so the control cannot prove the flame
3. Failed or cracked hot surface ignitor
4. Condensate or vent restriction interrupting the burner cycle

What you can safely check yourself

• Check that other gas appliances in the house (range, water heater) are working, which tells you whether gas is being delivered
• Check that the furnace door is fully seated, since the door switch must be closed for the furnace to fire
• Check the outdoor intake and exhaust pipes for snow, ice, leaves or nesting debris
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Flame sensor rod coated with oxide or scale
2. Poor unit ground, which lowers the measurable flame current
3. Burner or manifold problem producing a weak or lifting flame

What you can safely check yourself

• Check and replace the air filter, since poor airflow can distort the flame
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. No gas supply, or gas pressure too low to light the burners
2. Failed hot surface ignitor
3. Flame sensor unable to prove the flame after repeated lights
4. Vent or combustion air restriction cutting the ignition sequence short

What you can safely check yourself

• Check that other gas appliances in the house are working
• Check the outdoor intake and exhaust pipes for snow, ice or debris blockage
• Check that the furnace door is fully seated
• Turn the furnace power switch off for 30 seconds and back on once to clear the lockout, one time only

Likely causes, most common first

1. Dirty air filter restricting airflow across the heat exchanger
2. Closed, blocked or undersized supply and return registers
3. Blower not moving enough air, or a failed blower motor
4. Restricted or collapsed return duct

What you can safely check yourself

• Check the air filter and replace it if it is dirty
• Make sure supply and return registers throughout the house are open and not blocked by furniture or rugs
• Check that the return grille is not covered
• Turn the furnace power switch off for 30 seconds and back on once after the furnace has cooled

Likely causes, most common first

1. Blocked or restricted vent or flue
2. Combustion air inducer not moving enough air
3. Cracked or plugged heat exchanger
4. Blocked burners or misaligned burner flames

What you can safely check yourself

• Check the outdoor intake and exhaust pipes for snow, ice, leaves or nesting debris
• Check the air filter and replace it if it is dirty

Likely causes, most common first

1. Blocked or restricted intake or exhaust vent pipe
2. Plugged condensate drain or trap backing water into the pressure switch tubing
3. Cracked, kinked or disconnected pressure switch hose
4. Weak or failed combustion air inducer

What you can safely check yourself

• Check the outdoor intake and exhaust pipes for snow, ice, leaves or nesting debris
• Check the condensate drain line and trap for a clog or standing water
• Check that the furnace door is fully seated
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Partially blocked intake or exhaust vent
2. Plugged condensate drain or trap
3. Marginal or failing combustion air inducer
4. Loose or wet pressure switch tubing

What you can safely check yourself

• Check the outdoor intake and exhaust pipes for snow, ice or debris
• Check the condensate drain line and trap for a clog
• Turn the furnace power switch off for 30 seconds and back on once, one time only

Likely causes, most common first

1. Pressure switch stuck closed
2. Pressure switch tubing routed or connected incorrectly
3. Shorted pressure switch wiring

What you can safely check yourself

• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Gas valve leaking through and allowing gas to burn when it should be closed
2. Flame sense circuit or control board fault reading a false flame signal

What you can safely check yourself

• Turn the furnace power switch off at the switch beside the furnace

Likely causes, most common first

1. Utility supply voltage low or a brownout condition
2. Undersized or loose branch circuit wiring feeding the furnace
3. Loose connection at the disconnect or junction box

What you can safely check yourself

• Check whether other appliances and lights in the house are also acting up, which suggests a house-wide supply problem
• Check that the breaker feeding the furnace is fully on

Likely causes, most common first

1. Miswired or loose communication bus connections between thermostat, furnace and outdoor unit
2. A high voltage source of electrical noise near the system, such as a welder
3. Broken or damaged thermostat wiring

What you can safely check yourself

• Check that the thermostat display is on and, if it is battery powered, that the batteries are good
• Check that the furnace door is fully seated and the furnace power switch and breaker are on
• Turn power to the furnace off for 30 seconds and back on once

Likely causes, most common first

1. Utility supply voltage low or a brownout condition
2. Undersized or loose branch circuit wiring feeding the furnace

What you can safely check yourself

• Check whether lights and other appliances in the house are also dimming or misbehaving
• Check that the breaker feeding the furnace is fully on

Likely causes, most common first

1. Hot and neutral reversed at the furnace junction box
2. Reversed polarity in the branch circuit feeding the furnace

What you can safely check yourself

• Check that the furnace disconnect switch and breaker are on
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Ground wire missing, loose or corroded at the furnace
2. Furnace chassis not bonded to a proper earth ground

What you can safely check yourself

• Check that the furnace disconnect switch and breaker are on
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Utility supply voltage running high
2. Incorrect transformer tap or supply configuration at the building

What you can safely check yourself

• Check whether other equipment in the house is also affected

Likely causes, most common first

1. Additional power-robbing accessories connected to the 24 volt transformer
2. Undersized transformer for the installed system
3. Shorted or damaged low voltage wiring

What you can safely check yourself

• Check that the furnace door is fully seated
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Loose or broken thermostat wiring
2. Thermostat has lost power or locked up
3. Communication bus connection problem

What you can safely check yourself

• Check that the thermostat screen is on and set to Heat with the setpoint above room temperature
• Cycle power on the thermostat as the manual directs
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Hardware failure on the integrated furnace control board
2. Flame sense circuit fault on the board
3. Electrical noise or a power event that corrupted the control

What you can safely check yourself

• Turn the furnace power switch off for 30 seconds and back on once to cycle power on the control

Likely causes, most common first

1. Blocked or restricted vent or flue
2. Combustion air inducer not moving enough air
3. Cracked or plugged heat exchanger
4. Blocked burners causing flame to roll out of the burner box

What you can safely check yourself

• Check the outdoor intake and exhaust pipes for snow, ice, leaves or nesting debris
• Check the air filter and replace it if it is dirty

Likely causes, most common first

1. New or replacement control board installed without the unit size code being set
2. Invalid unit code entered during setup

Likely causes, most common first

1. Gas valve wiring connected incorrectly, usually after a service call or valve replacement
2. Loose or damaged gas valve wiring harness

Likely causes, most common first

1. Failed gas valve relay on the integrated control board
2. Incorrect wiring at the control or gas valve

Likely causes, most common first

1. Cracked or burned out hot surface ignitor
2. Loose or corroded ignitor connector
3. Broken ignitor wiring

What you can safely check yourself

• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Blocked or restricted intake or exhaust vent pipe
2. Plugged condensate drain or trap backing water into the system
3. Cracked, kinked or disconnected pressure switch hose
4. Weak or failing combustion air inducer

What you can safely check yourself

• Check the outdoor intake and exhaust pipes for snow, ice, leaves or nesting debris
• Check the condensate drain line and trap for a clog or standing water
• Check that the furnace door is fully seated
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Pressure switch stuck closed
2. Pressure switch tubing misrouted or blocked
3. Shorted pressure switch wiring

What you can safely check yourself

• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Blocked or restricted intake or exhaust vent pipe
2. Plugged condensate drain or trap
3. Cracked, kinked or disconnected high pressure switch hose
4. Combustion air inducer not reaching full speed

What you can safely check yourself

• Check the outdoor intake and exhaust pipes for snow, ice or debris
• Check the condensate drain line and trap for a clog
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. High pressure switch stuck closed
2. Pressure switch tubing misrouted or blocked
3. Shorted pressure switch wiring

What you can safely check yourself

• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Partially blocked intake or exhaust vent, including snow or ice at the termination
2. Plugged condensate drain or trap
3. Wet, cracked or loose pressure switch tubing
4. Failing combustion air inducer

What you can safely check yourself

• Check the outdoor intake and exhaust pipes for snow, ice, leaves or nesting debris
• Check the condensate drain line and trap for a clog or standing water
• Check that the furnace door is fully seated
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Low pressure switch failing to close on a normal low-fire start, often from a partially restricted vent
2. Continuous fan mode active at the start of the heat call, which is normal behavior

What you can safely check yourself

• Check the outdoor intake and exhaust pipes for snow, ice or debris if the code shows up repeatedly
• Check the air filter and replace it if it is dirty

Likely causes, most common first

1. Flame sensor rod coated with oxide or scale
2. Poor unit ground, which lowers measurable flame current
3. Weak or lifting burner flame

What you can safely check yourself

• Check and replace the air filter if it is dirty
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Gas valve leaking through and letting gas continue to burn
2. Flame sense circuit reading a false signal

What you can safely check yourself

• Turn the furnace power switch off at the switch beside the furnace

Likely causes, most common first

1. Dirty air filter restricting airflow across the heat exchanger
2. Closed, blocked or undersized supply and return registers
3. Blower not moving enough air
4. Furnace overfired, or blockage inside the heater

What you can safely check yourself

• Check the air filter and replace it if it is dirty
• Make sure supply and return registers throughout the house are open and not blocked by furniture or rugs
• Check that the return grille is clear
• After the furnace has cooled, turn the power switch off for 30 seconds and back on once

Likely causes, most common first

1. No gas supply to the furnace, or the manual gas valve is closed
2. Ignitor not lighting the burners
3. Flame sensor not proving the flame
4. Gas pressure too low

What you can safely check yourself

• Check that other gas appliances in the house are working, which tells you whether gas is being delivered
• Check that the furnace door is fully seated
• Turn the furnace power switch off for 30 seconds and back on once, one time only

Likely causes, most common first

1. Blocked or restricted intake or exhaust vent pipe
2. Plugged condensate drain or trap
3. Loose or wet pressure switch tubing
4. Failing combustion air inducer

What you can safely check yourself

• Check the outdoor intake and exhaust pipes for snow, ice or debris
• Check the condensate drain line and trap for a clog
• Turn the furnace power switch off for 30 seconds and back on once, one time only

Likely causes, most common first

1. Dirty flame sensor unable to hold the flame signal
2. Poor unit ground lowering flame current
3. Low or unstable gas pressure
4. Vent or condensate restriction cutting the cycle short

What you can safely check yourself

• Check that other gas appliances in the house are working
• Check the outdoor intake and exhaust pipes for snow, ice or debris
• Turn the furnace power switch off for 30 seconds and back on once, one time only

Likely causes, most common first

1. Dirty air filter restricting airflow
2. Closed or blocked supply and return registers
3. Blower not moving enough air
4. Furnace overfired or blockage inside the heater

What you can safely check yourself

• Check the air filter and replace it if it is dirty
• Make sure supply and return registers are open and unblocked
• After the furnace has cooled, turn the power switch off for 30 seconds and back on once

Likely causes, most common first

1. Failed or cracked hot surface ignitor
2. Failed ignitor triggering circuit on the control board
3. Loose or damaged ignitor wiring

What you can safely check yourself

• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Dirty air filter
2. Restricted, undersized or partially collapsed ductwork
3. Blower wheel dirty or blower performance degraded
4. Closed or blocked registers

What you can safely check yourself

• Check the air filter and replace it if it is dirty
• Make sure supply and return registers throughout the house are open and unblocked
• Check that the return grille is not covered

Likely causes, most common first

1. Seized blower motor bearings
2. Blower wheel stuck or jammed by debris
3. Failed blower motor module

What you can safely check yourself

• Check that the furnace door is fully seated
• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Failed or shorted discharge air temperature sensor
2. Damaged sensor wiring

What you can safely check yourself

• Turn the furnace power switch off for 30 seconds and back on once

Likely causes, most common first

1. Dirty air filter
2. Restricted or undersized ductwork
3. Closed or blocked registers

What you can safely check yourself

• Check the air filter and replace it if it is dirty
• Make sure supply and return registers throughout the house are open and unblocked
• Check that the return grille is not covered

Likely causes, most common first

1. Dirty air filter
2. Restricted or undersized ductwork pushing total external static pressure above 0.8 inches water column
3. Closed or blocked registers

What you can safely check yourself

• Check the air filter and replace it if it is dirty
• Make sure supply and return registers throughout the house are open and unblocked
• Check that the return grille is not covered