Error code reference
Lennox Furnace Error Code E120
Unresponsive device (communication only), usually caused by delay in outdoor unit responding to indoor unit…
Unresponsive device (communication only), usually caused by delay in outdoor unit responding to indoor unit polling.
First step from the service manual
1
Recycle power. Check all wiring connections.
The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.
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E120 on my Lennox Furnace
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Unresponsive device (communication only)
From Service Manual 799896, p.28:
1.Recycle power. Check all wiring connections.
2.Measure door lock assembly resistance across the PTC terminals. Expected: ~200 ohms cold, rising sharply once energized.
3.Inspect strike and latch alignment. A warped door or misaligned strike prevents the switch stack from closing in the correct sequence.
Part W10810403
Door lock assembly
Related Lennox furnace codes
All Lennox furnace codes →- 1Reverse line voltage polarity.
- 1 FlashFlame present with gas valve de-energized (control 103085).
- 1+2Low flame current during run mode (Board 100869).
- 1+3Flame sensed out of sequence
- 10Watchguard
- 11Pressure switch stuck closed prior to activation of…
- 2Improper Earth Ground.
- 2 FlashesPressure switch closed with combustion air inducer…
View all 213 Lennox furnace codes
- 2 Flashes (RAM)Control lockout
- 2+3Low fire pressure switch sensed open; 80%
- 2+4Low fire pressure switch sensed closed prior to activation…
- 2+5High fire pressure switch sensed open. 80%
- 2+6High fire pressure switch sensed closed prior to activation…
- 2+7Low pressure switch opened during ignition trial or heating…
- 3Burner failed to light, or lost flame during heat demand.
- 3 FlashesPressure switch open with combustion air inducer energized…
- 3 Flashes (RAM)Pressure switch open
- 3+1Limit switch open (primary or secondary).
- 4Low Flame Signal
- 4 FlashesPrimary limit switch open (Control 103085).
- 4 Flashes (RAM)Open limit
- 4+1Watchguard
- 4+2Watchguard
- 4+3Watchguard
- 4+5Watchguard
- 4+6Watchguard
- 4+7Watchguard
- 4+8Low line voltage (below 90 volts for board 100870).
- 5Watchguard
- 5 FlashesRollout switch open (control 103085).
- 5 Flashes (RAM)Flame sensed and gas valve not energized.
- 5+1Hard lockout
- 5+2Control failed self check, internal error; control will…
- 5+3No Earth ground; control will restart if error recovers…
- 5+4Reversed line voltage polarity; control will restart if the…
- 5+5Low 24V (control will restart if voltage rises to normal).
- 5+6Low secondary (24VAC) voltage (Board 100869).
- 6 FlashesPressure switch cycle lockout (control 103085).
- 7Primary or Secondary Limit Open or Watchguard Mode
- 7 FLASHESLockout
- 8Rollout switch open.
- 8 FlashesLockout
- 9Pressure Switch failed to close or opened during heat…
- 9 FlashesLine voltage polarity incorrect (Control 103085).
- DIAG#1: Alternate Flash / DIAG#2: Alternate FlashWatchguard
- DIAG#1: Continuous On / DIAG#2: Continuous OnCircuit board self-check failure or ignition/blower control…
- DIAG#1: Flash / DIAG#2: OffPressure switch Watchguard
- DIAG#1: Off / DIAG#2: FlashFlame sensed without gas valve energized.
- DIAG#1: On / DIAG#2: FlashPrimary or secondary limit switch open (auto-reset switch).
- DIAG#1: Simultaneous Flash / DIAG#2: Simultaneous FlashPower ON, normal operation; increased flash rate indicates…
- DS1 Alternating Fast Flash / DS2 Alternating Fast FlashDuring ignitor warm-up period only
- DS1 Alternating Slow Flash / DS2 Alternating Slow FlashWatchguard
- DS1 Fast Flash / DS2 Fast FlashNormal operation
- DS1 Fast Flash / DS2 Fast Flash (simultaneous)Normal operation
- DS1 Fast Flash / DS2 Slow FlashDuring ignitor warm-up period only
- DS1 OFF / DS2 Fast FlashHigh pressure switch open, or blocked inlet/exhaust vent…
- DS1 Off / DS2 OnLow flame signal; measures below 0.23 microAmps
- DS1 OFF / DS2 Slow FlashLow pressure switch open, or blocked inlet/exhaust vent, or…
- DS1 On / DS2 OffMain power polarity reversed; switch line and neutral.
- DS1 On / DS2 OnCircuit board failure or control wired incorrectly; check…
- DS1 Slow Flash / DS2 Fast FlashLow flame signal
- DS1 Slow Flash / DS2 OffFlame sensed without gas valve energized.
- DS1 Slow Flash / DS2 ONPrimary, secondary, backup secondary or rollout limit…
- DS1 Slow Flash / DS2 Slow FlashPower on
- DS1 Slow Flash / DS2 Slow Flash (simultaneous)Power on
- DS1/DS2 Alternating Fast FlashDuring ignitor warm-up only
- DS1/DS2 Alternating Slow FlashWatchguard
- E 111Line voltage polarity reversed.
- E 114Line voltage frequency out-of-range (no 60 Hertz power).
- E 117Poor ground detected (warning only).
- E 125Control failed self-check, internal error, failed hardware…
- E 226High pressure switch failed closed.
- E 227Low pressure switch open during trial for ignition or run…
- E 240Low flame current during run mode.
- E 250Limit switch circuit open.
- E 271Soft lockout
- E 272Soft lockout
- E 273Soft lockout
- E 274Soft lockout
- E 290Ignitor circuit fault
- E105Device communication problem
- E110Low line voltage detected (voltage lower than nameplate…
- E112Ground not detected; system shuts down.
- E113High line voltage detected (voltage higher than nameplate…
- E115Low 24V power (range is 18 to 30 volts); control will…
- E116High 24V (range is 18 to 30 volts).
- E124Active communicating thermostat signal missing for more…
- E126Control internal communication problem.
- E131Corrupted control parameters (communicating only).
- E180Outdoor air temperature sensor failure (shorted or…
- E200Device communication problem
- E201Indoor blower communication failure
- E202Indoor blower motor mis-match
- E203Appliance capacity/size is NOT programmed; invalid unit…
- E204Line voltage frequency out-of-range; no 60 hertz power.
- E205Low 24V
- E206Indoor blower motor mis-match
- E207Hot surface ignitor sensed open.
- E208Gas valve mis-wired.
- E209Gas valve control relay contact shorted.
- E210Unresponsive device; usually caused by delay in outdoor…
- E211Active communicating thermostat signal missing for more…
- E212Control failed self-check, internal error, failed hardware…
- E213Failed internal control communication between…
- E214Corrupted control parameters (verify configuration of…
- E215Outdoor air sensor failure
- E218Indoor blower communication failure
- E220Hard lockout
- E221Low pressure switch failed open.
- E222Low pressure switch failed closed.
- E223High pressure switch failed open.
- E224Low pressure switch failed closed (stuck closed on heat…
- E225Low pressure switch open during trial for ignition or run…
- E228Unable to perform successful pressure switch calibration.
- E229Ignition on High Fire
- E241Flame sensed out of sequence
- E251Discharge air temperature too high (gas heat only).
- E252Discharge air temperature too high (gas heat only).
- E260Soft lockout
- E261Soft lockout
- E262Soft lockout
- E263Soft lockout
- E264Soft lockout
- E265Soft lockout
- E266Soft lockout
- E269Ignitor circuit fault
- E270Restricted air flow
- E275Heat rate reduced to match indoor blower air flow; furnace…
- E276Restricted air flow in cooling or continuous fan mode is…
- E277Indoor or outdoor unit capacity mismatch.
- E278Global network connection
- E280No 24 Volt output on Y1 to C with non-communicating outdoor…
- E281No 24 Volt output on Y2 to C with non-communicating outdoor…
- E282No 24 Volts between R & O with non-communicating outdoor…
- E291Heat air flow restricted below the minimum.
- E292Indoor blower motor unable to start due to obstructed wheel…
- E294Combustion air inducer over current.
- E295Indoor blower motor temperature is too high (motor tripped…
- E300LSOM
- E301LSOM
- E302LSOM
- E303LSOM
- E304LSOM
- E305LSOM
- E306LSOM
- E307LSOM
- E308LSOM
- E310Discharge air temperature sensor failure (shorted or out of…
- E311Heat rate reduced to match indoor blower air flow (furnace…
- E312Restricted air flow in cooling or continuous fan mode is…
- E313Indoor or outdoor unit capacity mismatch (communication…
- E331Global network connection
- E334Relay Y1 stuck on integrated control.
- E347No 24 Volt output on Y1 of integrated control with…
- E348No 24 Volt output on Y2 of integrated control with…
- E349No 24 Volts between R & O on integrated control with…
- E370Interlock switch sensed open for 2 minutes.
- E400LSOM
- E401LSOM
- E402LSOM
- E403LSOM
- E404LSOM
- E405LSOM
- E406LSOM
- E407LSOM
- E408LSOM
- E409LSOM
- Fast Flash (RAM)Normal operation with call for heat (RAM control).
- HeartbeatNormal operation
- LED Flashing 1/2 sec on 1/2 sec offLockout or Restricted Lockout condition on the R7184B…
- LED Flashing 2 sec on 2 sec offBurner control is in Recycle mode
- LED OnFlame sensed by the R7184B burner control (cad cell…
- LED Slow FlashControl retry period
- LED Steady OnNormal operation of the G776 ignition control (-1 through…
- LED-OFFControl failure or power failure or hard lockout (G776…
- LED-SLOW_FLASH_1ON_5OFFControl retry period
- LED-STEADY_ONNormal operation (G776 ignition control, -1 and -2 models).
- LED#1 Alternating Fast Flash / LED#2 Alternating Fast FlashSensed during ignitor warm-up period only
- LED#1 Alternating Slow Flash / LED#2 Alternating Slow FlashWatchguard
- LED#1 Fast Flash / LED#2 Fast Flash (simultaneous)Normal operation, signaled when heating demand initiated at…
- LED#1 Fast Flash / LED#2 Slow FlashMain power polarity reversed
- LED#1 OFF / LED#2 ONCircuit board failure or control wired incorrectly.
- LED#1 Off / LED#2 Slow FlashPressure switch open or has opened 5 times during a single…
- LED#1 ON / LED#2 OFFCircuit board failure or control wired incorrectly.
- LED#1 ON / LED#2 ONCircuit board failure or control wired incorrectly.
- LED#1 ON / LED#2 Slow FlashRollout switch open, or 12-pin connector improperly…
- LED#1 Simultaneous Fast Flash / LED#2 Simultaneous Fast FlashNormal operation
- LED#1 Simultaneous Slow Flash / LED#2 Simultaneous Slow FlashPower on, normal operation; also signaled during cooling…
- LED#1 Slow Flash / LED#2 Fast FlashLow flame signal
- LED#1 Slow Flash / LED#2 OFFFlame sensed without gas valve energized.
- LED#1 Slow Flash / LED#2 ONPrimary or secondary limit open; limit must close within…
- LED#1 Slow Flash / LED#2 Slow Flash (simultaneous)Power on, normal operation; also signaled during cooling…
- LED1 ALT FAST FLASH / LED2 ALT FAST FLASHDuring ignitor warm-up
- LED1 ALT SLOW FLASH / LED2 ALT SLOW FLASHWatchguard
- LED1 FAST FLASH / LED2 FAST FLASHNormal operation
- LED1 SLOW FLASH / LED2 SLOW FLASHPower on
- LED1-ALT_FAST_FLASH/LED2-ALT_FAST_FLASHImproper main ground or line voltage below 75 volts; OR…
- LED1-ALT_SLOW_FLASH/LED2-ALT_SLOW_FLASHWatchguard
- LED1-FAST_FLASH/LED2-FAST_FLASHNormal operation
- LED1-FAST_FLASH/LED2-SLOW_FLASHMain power polarity reversed
- LED1-OFF/LED2-SLOW_FLASHPressure switch open or has opened 5 times during a single…
- LED1-ON/LED2-SLOW_FLASHRollout switch open OR 9-pin connector improperly attached.
- LED1-SLOW_FLASH/LED2-FAST_FLASHLow flame signal
- LED1-SLOW_FLASH/LED2-OFFFlame sensed without gas valve energized.
- LED1-SLOW_FLASH/LED2-ONPrimary or secondary limit open. Units with board 63K8901…
- LED1-SLOW_FLASH/LED2-SLOW_FLASHPower on
- OffNo power to control or board fault detected.
- OnBoard fault detected.
- PulsePower on
- SIMULTANEOUS FAST FLASHNormal operation
- SIMULTANEOUS SLOW FLASHPower on
- Slow Flash (RAM)Normal operation, no call for heat (RAM control).
- Steady ON (RAM)Control board failure (RAM control).
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