Home / Guided troubleshooting / Furnace not heating

Furnace not heating: guided troubleshooting

Gas furnace with a call for heat and no warm air. Answer each question from what you see on site.

Step 1

Is the thermostat display on and calling for heat?

For trained technicians. Line voltage, gas and refrigerant work can injure or kill. Never jumper a safety switch to keep equipment running, and follow the manufacturer's instructions for the unit in front of you.
See every step in this flow
  1. Is the thermostat display on and calling for heat?
    • Yes: Look at the LED or display on the furnace control board. What does it show?
    • No, it's blank or dead: Start at the thermostat and the 24 V supply
  2. Look at the LED or display on the furnace control board. What does it show? Read it through the sight glass before opening the door or cycling power; a reset can clear the code.
    • A fault code: Look up the code
    • Nothing, no light at all: No power to the board
    • Normal, no fault: On a call for heat, does the inducer (draft) motor start?
  3. On a call for heat, does the inducer (draft) motor start?
    • Yes: Does the igniter glow, or the spark igniter spark?
    • No: Inducer isn't starting
  4. Does the igniter glow, or the spark igniter spark?
    • Yes: Do the burners light?
    • No: No ignition attempt
  5. Do the burners light?
    • Yes: Do the burners stay lit?
    • No: Igniter works, no flame
    • They light late, with a boom: Delayed ignition
  6. Do the burners stay lit?
    • They go out within a few seconds: Flame isn't being sensed
    • They run a few minutes, then shut off: Tripping on high limit
    • They stay lit: Does the blower come on, and is the air warm?
  7. Does the blower come on, and is the air warm?
    • Blower doesn't start: Blower isn't starting
    • Blower runs but air is cool: Running, but the air is cool

Where each path ends

Start at the thermostat and the 24 V supply

A blank or dead thermostat usually means no 24 V: dead batteries, a tripped breaker or service switch, an open door switch, a blown low-voltage fuse on the board, or a tripped condensate float switch.

Look up the code

The code names the circuit that failed. Look it up for the exact series, then test that circuit.

No power to the board

Check the service switch, breaker and door switch for 115 V, then the transformer and low-voltage fuse for 24 V.

Inducer isn't starting

Check for voltage at the inducer on a call for heat. Voltage and no rotation is the motor; no voltage is the board, or a pressure switch stuck closed before the cycle starts.

No ignition attempt

If the inducer runs and nothing follows, the pressure switch probably isn't proving draft. If the switch closes and the igniter still doesn't glow, test the igniter.

Igniter works, no flame

Check for 24 V at the gas valve during the trial for ignition, then gas supply and pressure.

Delayed ignition

Gas is building up before it lights. Don't keep cycling it. Check burners and crossovers, igniter position and gas pressure.

Flame isn't being sensed

Burners that light and drop out in a few seconds point at the flame sensing circuit: a dirty sensor, a poor ground or a weak flame at the sensor.

Tripping on high limit

Usually airflow: filter, blower speed, closed registers or a dirty coil. Measure temperature rise against the nameplate. Never jumper a limit.

Blower isn't starting

Check for voltage at the blower after the heat-on delay, then the capacitor or motor module.

Running, but the air is cool

Check temperature rise, duct leakage in unheated spaces, and whether the burners are cycling on limit behind the scenes.