Furnace not heating: common causes
A furnace is a sequence of steps: the thermostat calls for heat, the inducer fan starts, the ignitor lights the burners, a flame sensor confirms the fire, and the blower pushes warm air through the ducts. A failure at any step stops the whole chain, so the symptom points to where to look.
- Cold air from vents: the burners did not stay lit, often a dirty flame sensor or an ignitor that has weakened.
- Furnace runs but air is weak: a clogged filter, a blocked return or a failing blower motor.
- Starts, runs briefly, shuts off: overheating from restricted airflow, tripping the limit switch.
- No response at all: thermostat batteries, a tripped breaker, a switch at the unit or a blown fuse on the control board.
- Clicking with no flame: ignition or gas supply trouble.
- Rattling or squealing: loose panels, a worn belt or a failing bearing.
Furnace troubleshooting and diagnosis
A few checks are reasonable before calling. Confirm the thermostat is set to heat and above room temperature, replace its batteries if it uses them, check that the breaker is on, and look at the filter. A filter packed with dust is one of the most common reasons for poor airflow and shutdowns.
Stop there. Beyond those checks, a technician works with a meter, a manometer and the control board error codes. Many furnaces flash a coded light that records the last fault, and the access panel usually lists what the code means. Diagnosis then confirms the part that failed rather than guessing, since replacing parts at random costs time and sometimes hides the real fault, such as a cracked heat exchanger or a poor vent.
- Check the thermostat, batteries and breaker.
- Replace a dirty filter and make sure vents are open.
- Note any error light pattern or noise, and when the problem happens.
- Call a technician if the furnace still does not run.
Gas furnace repair and safety
If you smell natural gas, leave the building immediately and call the gas utility or 911 from outside. Do not flip switches or use a phone inside. For carbon monoxide, keep working detectors on each level and near sleeping areas, and treat any alarm as real: get everyone outside and call for help.
Do not remove burner covers, adjust gas valves or relight a pilot unless the manufacturer instructions explicitly describe how and you are comfortable doing so. A cracked heat exchanger or blocked flue can leak combustion gases into the house, which is why technicians check venting and combustion, not only whether the furnace turns on. Equipment at higher elevation may also need adjustment under the manufacturer instructions, something a technician should confirm.
When to repair versus replace
A failed ignitor, sensor, capacitor or motor is a normal repair, and a well-kept furnace can run for many years. Replacement starts to make sense with a cracked heat exchanger, repeated breakdowns in one season, rising energy use, uneven heating, or a unit well past its expected lifespan. Think about the cost of the repair relative to the age of the furnace, and ask the technician for the condition of the heat exchanger before deciding.
Our guide on repairing versus replacing a furnace walks through the decision in more detail.
Why call Rocket
Rocket is a locally owned father-and-sons team from West Jordan. In a cold snap, you can call or text any time, since emergency hours are listed as 24/7.
Frequently asked questions
Why is my furnace blowing cold air?
Often the burners lit and then shut off, because of a dirty flame sensor or an overheating limit. It can also be a thermostat fan setting. A technician can tell which.
How often should I change the furnace filter?
Follow the filter and furnace instructions, and check it monthly in winter. Dust, pets and remodeling shorten its life.
Is it safe to relight a pilot light?
Only if the manufacturer instructions for your furnace say so and you are confident. If the pilot keeps going out, have it inspected.
What does a flashing light on my furnace mean?
It is a diagnostic code. The panel usually has a chart. Note the pattern and share it when you call.
My furnace has no heat in freezing weather, what should I do?
Check the thermostat and breaker, then call. Open cabinet doors under sinks on exterior walls, to reduce pipe freeze risk while you wait.
Is there a diagnostic fee for a furnace repair visit?
Rocket charges an $89 trip charge to diagnose a furnace problem. If Rocket does the work during that visit, the charge is waived. If the repair is scheduled for another day, the $89 is credited toward the repair price.
