Furnace troubleshooting guides

97 guides. No heat, short cycling, ignition and blower problems on gas and electric furnaces. Each one lists the likely causes and what to check, in order.

All brands (43)

• Blower cycles on and off
  Possible causes and checks for: blower cycles on and off.
• Blower runs, not enough heat
  Possible causes and checks for: blower runs, not enough heat.
• Blowing cold air
  The blower runs but the air coming out of the registers is cold.
• Board shows "flame sensed with no call for heat"
  Possible causes and checks for: board shows "flame sensed with no call for heat".
• Can't pull oil (distance/lift)
  Possible causes and checks for: can't pull oil (distance/lift).
• CO in flue / poor combustion numbers
  Low CO2 + high O2 + CO → excess air too high or manifold pressure low; high CO2 + low O2 + CO → air shortage or manifold pressure high.
• Cold air blast at start / blower never stops
  Possible causes and checks for: cold air blast at start / blower never stops.
• Combustion gas / odour in house, flu-like symptoms
  Possible causes and checks for: combustion gas / odour in house, flu-like symptoms.
• Cycles on limit (not lockout)
  Possible causes and checks for: cycles on limit (not lockout).
• Does not start
  Possible causes and checks for: does not start.
• Flame burns a few seconds then out
  Possible causes and checks for: flame burns a few seconds then out.
• Flame conditions table
  Possible causes and checks for: flame conditions table.
• Heat exchanger leak
  Possible causes and checks for: heat exchanger leak.
• High blower-door ACH50 (>3)
  Possible causes and checks for: high blower-door ach50 (>3).
• High fuel consumption
  Possible causes and checks for: high fuel consumption.
• High stack temperature
  Possible causes and checks for: high stack temperature.
• High stack temperature / low efficiency
  Possible causes and checks for: high stack temperature / low efficiency.
• Indoor blower runs, outdoor unit dead
  Possible causes and checks for: indoor blower runs, outdoor unit dead.
• Limit opens
  Possible causes and checks for: limit opens.
• Locks out — oil but no ignition
  Possible causes and checks for: locks out — oil but no ignition.
• Low CO2 (<8 %)
  Possible causes and checks for: low co2 (<8 %).
• Low draft / blowback
  Possible causes and checks for: low draft / blowback.
• Modulating furnace won't modulate
  Possible causes and checks for: modulating furnace won't modulate.
• Negative house pressure, backdrafting, infiltration
  Possible causes and checks for: negative house pressure, backdrafting, infiltration.
• No heat
  Thermostat is calling for heat but the furnace does not start or does not light.
• No heat
  Possible causes and checks for: no heat.
• No heat (ignition stage)
  Generic sequence
• Noisy
  Possible causes and checks for: noisy.
• Noisy registers, drafts, blower overamps/ECM noise, coil freezing
  Possible causes and checks for: noisy registers, drafts, blower overamps/ecm noise, coil freezing.
• Not running / fault code
  Possible causes and checks for: not running / fault code.
• Oil + ignition OK but no flame
  Possible causes and checks for: oil + ignition ok but no flame.
• Oil on floor
  Possible causes and checks for: oil on floor.
• Pressure switch open / stuck
  Possible causes and checks for: pressure switch open / stuck.
• Puff-back / delayed ignition
  Possible causes and checks for: puff-back / delayed ignition.
• Reduced airflow, hot motor
  Possible causes and checks for: reduced airflow, hot motor.
• Reduced pressure, smoky shutdowns, lockouts
  Possible causes and checks for: reduced pressure, smoky shutdowns, lockouts.
• Rollout switch tripped
  Possible causes and checks for: rollout switch tripped.
• Short cycling
  The furnace lights and runs for a short time, shuts off, and repeats.
• Smoke, soot, odour, pulsation
  Possible causes and checks for: smoke, soot, odour, pulsation.
• Starts but locks out — no oil at nozzle
  Possible causes and checks for: starts but locks out — no oil at nozzle.
• Too little / too much air
  Possible causes and checks for: too little / too much air.
• Unsatisfactory heat (short cycles / uneven / not enough)
  Possible causes and checks for: unsatisfactory heat (short cycles / uneven / not enough).
• Water flows with furnace off
  Possible causes and checks for: water flows with furnace off.

Carrier (2)

• Burners will not light or stay lit - Carrier 58SB0A/58SB1A
  Ignition-proving problems (codes 34 and 14) worked through with the component test.
• Furnace not responding to thermostat - Carrier 58SB0A/58SB1A
  No heat or cool response although the control has power. Flowchart-based checks for the single-stage 58SB control.

Daikin (3)

• Control display blank, furnace dead - Daikin DR96TN/DD96TN
  The furnace does nothing and the 7-segment display on the integrated control module shows nothing (the troubleshooting chart lists this as LED status
• Furnace not paired with R-32 cooling - disable A2L function - Daikin DR96TN/DD96TN
  The DR96TN/DD96TN control ships with the R-32 (A2L) leak-mitigation function ON. If the furnace is used with a non-R-32 cooling system or no sensor is
• No heat - Daikin DR96TN/DD96TN
  Thermostat calls for heat but the furnace does not start, light or deliver heat. Step-by-step check of the heating sequence from the manufacturer flow

Goodman (6)

• ComfortNet data line check - Goodman *MVC95/*CVC9
  How to check the two communication wires (data 1 and data 2) when the furnace, thermostat and outdoor unit are not talking.
• Control display blank, furnace dead - Goodman/Amana 9-speed ECM furnaces
  The furnace does nothing and the 7-segment display on the integrated control module shows nothing (the troubleshooting chart lists this as LED status
• Display blank, furnace dead - Goodman *MVC95/*CVC9
  The two-digit display shows nothing and the furnace does nothing. A ComfortNet thermostat may show INTERNAL FAULT EE with Call for Service.
• No heat - Goodman/Amana 9-speed ECM furnaces
  Thermostat calls for heat but the furnace does not start, light or deliver heat. Step-by-step check of the heating sequence from the manufacturer flow
• Poor combustion / flame problems - Goodman/Amana GM9C96
  Burner flame is unstable, lifting, yellow or the furnace has repeated flame or pressure switch problems with no single clear code. The manual lists th
• Temperature rise out of range - Goodman *MVC95/*CVC9
  Supply air too hot or too cold compared to the rating plate rise range.

ICP (4)

• Check inlet gas pressure
  Low inlet gas pressure prevents setting the right input and causes ignition/pressure switch faults.
• Component self-test
  Runs inducer, gas valve communication check, igniter and blower to isolate a failed part.
• No heat - control does not respond to thermostat
  LED heartbeat, no stored codes, but furnace does not respond.
• Reading and retrieving status codes
  Status LED: heartbeat (bright-dim) = normal; code = short flashes (first digit) then long flashes (second digit). Up to 7 codes are stored and survive

Johnson Controls (1)

• No heat - SmartValve SV9501/SV9502 pilot does not light or main burner does not come on
  Step-by-step check of the intermittent-pilot SmartValve ignition on RGAC/SGAC units.

KeepRite (4)

• No heat, control ignores the thermostat - KeepRite (F/G)9MXE
  Status light shows a heartbeat but the furnace does not respond to W, Y or G.
• No heat, control ignores the thermostat - KeepRite (F/G)9MXT
  Status light shows a heartbeat but the furnace does not respond to W, Y or G.
• Reading stored codes and component test - KeepRite (F/G)9MXE
  How to recall the last fault and run the built-in component test.
• Reading stored codes and component test - KeepRite (F/G)9MXT
  How to recall the last fault and run the built-in component test.

Lennox (12)

• Blower motor does not run - Lennox SL280UHV ECM checks
  Variable-speed blower does not run; winding checks from the SL280UHNV service manual.
• Display shows two horizontal bars (soft disable) - Lennox SL297NV
  Two horizontal bars on the 7-segment display mean the iComfort thermostat found a device on the RSBus it does not recognize and put it in soft disable
• EL296UHV: normal gas heat sequence (what should happen after a call for heat)
  Use this to see where a stuck furnace is stopping. Times are from the manual for this series.
• EL296UHV: what the display and the diagnostic button are telling you
  The furnace control has a seven-segment display and one push button. Normal operation, stored errors, flame current and the unit size are all read fro
• Furnace will not run (no heat) - Lennox EL195UHNE
  Quick checks from the manual when the EL195UHNE does not start on a call for heat.
• Furnace will not run (no heat) - Lennox SL297NV
  Basic checks from the manual when the SL297UHNV does not operate.
• Furnace will not run - Lennox SL280UHV checklist
  SL280UHV / SL280UHNV furnace does not start on a heat call; basic checks from the installation manual before deeper diagnosis.
• Indoor blower does not run - Lennox EL195UHNE
  Constant-torque ECM blower motor check on the EL195UHNE. The motor needs 120 VAC on J48 and 24 VAC (12-33 VAC) on one speed tap of J49.
• Indoor blower does not run - Lennox ML180UHE
  Constant-torque ECM blower motor check on the ML180UHEK. The motor needs 120 VAC power on J48 and 24 VAC (12-33 VAC) on one speed tap of J49.
• Poor ignition or combustion resonance - Lennox EL195UHNE
  Burner is slow/hard to light or runs with a loud resonance (rumble/howl). On the EL195UHNE this is usually the air orifice in the air intake coupling,
• Poor ignition or loud combustion resonance - Lennox SL280UHNV
  SL280UHNV (ultra-low NOx air/gas burner) has trouble lighting or lights with a loud resonance. Hard ignition means too much combustion air (air orific
• Two horizontal lines on display (soft disable) - Lennox communicating
  The control shows two horizontal lines on its 7-segment display (damper module or EIM: green LED 3 s on / 1 s off). The thermostat found a device it d

Midea (2)

• No light and no fan - furnace does not start
  Furnace shows no LED activity and the blower does not run on a call for heat or fan.
• Resetting a locked-out furnace
  How to clear a lockout after ignition failure on the BGH96M control.

Rheem (19)

• Blower codes 66, 61 and 60 - what to look for
  Quick sorting of the three ECM motor codes on the R96V control.
• Blower does not start after burners light
  R95P: blower should start on heat speed about 20 seconds after burners light.
• Burners go out before the heat call ends / short cycling
  Main burners do not stay lit until the thermostat is satisfied, or the room temperature is not held near setpoint.
• Communicating system not found / no communication
  RGPE/RGLE: thermostat should show SEARCHING then FURNACE FOUND and AIR CONDITIONER/HEAT PUMP FOUND within 30 minutes.
• ECM blower does not start on heat
  RGPE/RGLE: blower should start on high heat speed 15-20 seconds after burners light.
• ECM blower will not run - power and communication checks (code 68)
  The communicating blower motor does not start on a heat, cool or fan call and the control shows 68.
• Furnace no heat - sequence troubleshooting
  Step through the normal heat sequence: pressure switch check, 30-second prepurge, 8-second ignition trial on low fire, blower on about 20 seconds afte
• Furnace short cycles or does not hold setpoint
  Heat cycles end early or room temperature drifts.
• Gas valve does not shut off when heat call ends
  Burners stay lit after the thermostat is turned off.
• Heat call troubleshooting walk-through (two-stage communicating IFC)
  Order of checks from the factory diagnostic chart when a heat call does not end in high-fire heat.
• Indoor blower does not start after burners light
  Burners light but the main blower does not start on heat speed about 20 s later.
• Inducer (draft motor) does not start on heat call
  R95P: no inducer on a heat call.
• Inducer does not run / does not run on low speed
  RGPE/RGLE 2-stage.
• Inducer runs 60 s then shuts off for 5 minutes, or runs continuously
  Inducer cycles on 60 s / off 5 min, or never stops, without the furnace lighting.
• Inducer runs 60 seconds then off for 5 minutes
  R95P: pressure switch not proving.
• No heat - inducer (IDM) does not start
  POWER LED on, thermostat calling, but the induced draft motor never starts.
• No heat - POWER LED off
  Furnace dead on a heat call and the IFC POWER LED is not lit.
• Read target airflow from the CFM LED
  RGRM/RGTM controls have a CFM LED that blinks out the airflow the blower is aiming for. Use it as a rough check of blower setup, not as a measurement.
• Thermostat does not hold room temperature
  R95P: furnace runs but room temp not near set point.

Trane (1)

• Reading and clearing fault history - Trane S-Series IFC
  How to pull up stored faults, wipe them and reset the furnace control to factory values with the Menu and Option buttons.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.