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Furnace not turning on: causes, fixes and what it costs
A furnace that will not start is giving you more information than it seems. Counting what happens before the silence, and whether anything moves at all, narrows the diagnosis considerably.
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Call (855) 321-3194Is this an emergency or can it wait?
Set the thermostat to off so it stops retrying, then check the breaker, the furnace service switch and the filter. If none of those is the cause, call (855) 321-3194 and we connect you with a local heating provider who reads the lockout code.
What you are seeing
What the furnace does
- No response at all when heat is called for
- The inducer runs but the burner never lights
- Three ignition attempts then a lockout
- An error code flashing at the control board
What to check first
- The thermostat screen is blank
- The furnace service switch has been knocked off
- The breaker has tripped
- The filter is heavily loaded
Do and do not
| Do | Do not |
|---|---|
| Set the thermostat to off between attempts | Reset the furnace repeatedly hoping it catches |
| Check the service switch beside the furnace | Assume the switch is a light switch and ignore it |
| Note any flashing error code and its pattern | Clear the code before anyone has read it |
| Replace a loaded filter | Run the furnace with the blower door removed |
| Leave the building and call 911 if you smell gas | Try to relight anything after smelling gas |
What causes it, ranked by how often
- Dirty flame sensor. The furnace lights then shuts down within seconds because the sensor cannot confirm the flame. After an idle summer this is the single most common no-heat call, and it is a cleaning job.
- Failed igniter. No ignition at all, though the inducer runs and the sequence starts. Igniters are consumable and fail at predictable intervals.
- Tripped limit switch. Restricted airflow makes the furnace overheat and shut itself down as designed. The root cause is usually a blocked filter or closed registers.
- Pressure switch or blocked flue. If the furnace cannot prove adequate draft it will not open the gas valve. Often a blocked intake or exhaust terminal.
- Power or switch off. A tripped breaker, a blown low-voltage fuse on the control board, or the furnace service switch knocked off during storage.
- Thermostat fault. A control with dead batteries or a broken conductor never asks the furnace to run at all.
How a technician diagnoses it
- Confirm power and the call for heat
Breaker, service switch, board fuse and a thermostat actually closing the circuit.
- Read the error code
Modern boards flash a diagnostic pattern that names the failed step in the sequence.
- Watch the start sequence
Inducer, pressure switch, igniter, gas valve, flame sensor. Whichever step stalls is the fault.
- Test the flame signal
Measured in microamps. A weak signal confirms a dirty or failing sensor rather than a gas supply problem.
When to shut the system off
- You smell gas at any point. Leave the building and call the gas utility or 911 from outside.
- A carbon monoxide alarm is sounding. Leave and call 911.
- There is a burning or electrical smell from the cabinet.
- Soot marks are visible around the burner compartment.
Service pages
Related guides
Safety first
Call now, or call the utility first?
Call now
- No cool air during a heat advisory
- No heat with outdoor temperatures below freezing
- Burning smell from the vents
- Water pooling at the air handler
- Ice on the line set
- Breaker tripping on every cycle
Call (855) 321-3194 and give your ZIP code.
Shut it off and call the utility or 911 first
- You smell gas: leave the building, then call the gas utility or 911
- Smoke or sparking at the equipment
- A carbon monoxide alarm is sounding: leave and call 911
Reference: CPSC recalls and US Fire Administration heating fire safety.
Questions
Furnace not turning on questions
Why does my furnace try three times then stop?
That is a designed lockout. The board attempts ignition a set number of times, and if the flame sensor never confirms a flame it shuts the gas off for safety. A dirty flame sensor is the usual cause.
Can I clean the flame sensor myself?
It is possible but it means opening a gas appliance, and getting the reassembly wrong has real consequences. The cleaning itself is minutes of work for a technician and is usually bundled into a service visit.
Why would the filter stop my furnace?
A blocked filter restricts airflow across the heat exchanger, which overheats and trips the high limit switch. The furnace shuts down to protect itself. It reads as a failure but it is the safety working.
What does a flashing light on the furnace mean?
It is a diagnostic code. The pattern of flashes maps to a specific fault, and the legend is usually printed inside the furnace door. Tell the provider the pattern when you call.
Is a furnace that will not start dangerous?
A furnace that will not light is generally safer than one running badly. The dangerous situations are a gas smell, a sounding carbon monoxide alarm, or soot around the burners. Any of those means leaving the building first.
How much does a no-heat call cost?
A diagnostic runs a figure the provider states up front. A flame sensor clean is a figure the provider confirms on site an igniter a figure the provider confirms on site and a control board a figure the provider confirms on site. The provider confirms the figure before fitting anything.
Written and fact-checked by the HVAC Emergency USA editorial team. Last updated . Editorial policy.