Furnace Not Working? The No-Heat Troubleshooting Checklist
• By the Citywide Express HVAC technical team
The biggest no-heat spike of the year happens in the first two weeks of real cold, when systems that sat idle for months are suddenly asked to run flawlessly. Here is how to triage it before you call.
Step 1: Confirm it's actually calling for heat
- Thermostat set to HEAT, setpoint above room temperature, fresh batteries.
- Check the furnace switch on the wall or unit — it looks like a light switch and gets flipped off constantly.
- Check the breaker, and make sure the furnace door panel is fully seated (a loose door opens a safety switch).
Step 2: Airflow and filter
A filter left in since spring is the number one cause of furnace short cycling and limit-switch lockouts. Restricted airflow overheats the heat exchanger, the high limit trips, and the furnace shuts down after a few minutes of heat. Replace the filter, open all vents, and see if the cycle normalizes.
Step 3: Read the blink code
Most modern furnaces flash a diagnostic code through a small window. Count the blinks and check the legend inside the door.
- Ignition lockout — failed hot surface ignitor or dirty flame sensor.
- Pressure switch open — blocked flue, condensate clog, or failing inducer motor.
- Limit switch open — airflow restriction, dirty blower, closed vents.
- Flame sensed with no call for heat — gas valve or board fault. Shut it down and call.
What a proper heating repair visit includes
- Full sequence-of-operation test, not just a parts swap
- Gas pressure and temperature-rise measurement
- Flame sensor cleaning and ignitor resistance check
- Static pressure and blower amp draw
- Carbon monoxide test at the supply plenum
- Written findings and upfront repair pricing
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