Direct answer: When a furnace starts, lights, and then shuts off, the most common homeowner-level causes are a dirty furnace filter, thermostat or door-switch issues, or a dirty furnace flame sensor. If it tries to light repeatedly, smells like gas, or shuts down with unusual noises, stop and call for service.
Most likely: Start by separating a simple airflow or control problem from a combustion safety shutdown. A furnace that runs for only a few seconds after ignition often points to flame sensing. A furnace that runs a minute or two and quits can be airflow, overheating, or venting.
Watch one full startup cycle from the thermostat call to shutdown. That pattern tells you more than the symptom name does. Reality check: many furnaces that 'won't stay lit' are actually shutting themselves down on purpose. Common wrong move: changing parts before checking the filter, blower door, and vent termination outside.
Don’t start with: Do not start by replacing the gas valve, control board, or pressure switch. Those are expensive guesses and not safe first moves on a gas furnace.
You hear the inducer and ignition sequence, the burners come on, then the flame drops out quickly while the blower may or may not start.
Start here: Start with the flame sensor, blower door fit, and filter condition.
The furnace seems to heat briefly, then cuts out and may restart again soon.
Start here: Start with airflow restriction, closed registers, and signs of overheating.
You hear repeated ignition attempts, then the furnace sits idle until power is reset or the thermostat cycles.
Start here: Treat this as a safety lockout. Check the easy external items only, then stop if the cause is not obvious.
Air may move through the house, but the burners do not stay lit long enough to produce steady heat.
Start here: Separate a blower or thermostat issue from a combustion issue before assuming the furnace itself needs major parts.
A clogged filter, too many closed supply registers, or a blocked return can overheat the heat exchanger area and trip a limit control, especially if the furnace runs a little before shutting down.
Quick check: Pull the furnace filter and inspect it in good light. If it is gray, packed, or bowed, replace it with the same size and airflow rating.
If the burners light normally and then drop out within a few seconds, the furnace may not be proving flame even though ignition happened.
Quick check: Watch the burner flame through the sight glass. If ignition is clean but the flame cuts out fast, a dirty furnace flame sensor is a strong possibility.
A loose lower panel or weak door-switch contact can interrupt the control circuit and make the furnace act erratic or shut down mid-cycle.
Quick check: Press on the blower door and confirm the panel sits flat and tight with all tabs engaged.
High-efficiency furnaces can shut down if the vent termination is blocked by debris, snow, ice, or nesting material. Other safety faults can look similar.
Quick check: From outside, inspect the furnace vent openings for obvious blockage only. Do not disassemble vent piping.
You need to separate a quick flame-dropout from a later overheat shutdown. Those look similar from the hallway but lead to different fixes.
Next move: If the furnace now runs a full heating cycle and the house warms normally, the issue may have been a loose panel, thermostat setting, or an intermittent airflow problem. Keep checking the simple items below so it does not come right back. If the burners light and drop out within seconds, go to the flame-sensor and panel checks next. If it runs longer before quitting, focus on airflow and venting first.
What to conclude: The timing of the shutdown is the best clue. Seconds points more toward flame proving or control interruption. A minute or two points more toward overheating or another safety trip.
These are fast, safe checks that can mimic a furnace fault and waste a lot of time if you skip them.
Next move: If the furnace runs normally after reseating the door or correcting thermostat settings, you likely had a control interruption rather than a failed combustion part. If nothing changes, move to airflow checks next. If pressing on the door changes the behavior, the door switch or panel fit needs closer attention.
What to conclude: A furnace cannot run reliably if the control circuit is being interrupted by a loose door switch or bad call for heat.
Restricted airflow is one of the most common reasons a furnace starts, heats briefly, and shuts back down. It is also the safest thing to correct first.
Next move: If the furnace now stays on and heats normally, the shutdown was likely an overheat response caused by poor airflow. If the burners still drop out within a few seconds, the filter was not the main issue. Continue to the flame-sensor branch. If it runs longer but still quits, venting or a deeper limit issue is more likely.
A dirty furnace flame sensor is one of the few common, specific causes that fits the classic pattern of burners lighting and then shutting off within seconds.
Next move: If the burners now stay lit and the furnace completes a normal cycle, the furnace flame sensor was likely dirty and is the supported repair path. If the flame still drops out fast, stop short of deeper combustion diagnosis. The problem may involve ignition, grounding, venting, condensate, wiring, or controls that are not good guess-and-buy territory.
Once the simple control, filter, and flame-sensor checks are done, the remaining causes move into higher-risk furnace safety territory.
A good result: If clearing an obvious vent blockage restores normal operation, keep monitoring the furnace for a full day and check that the vent area stays clear.
If not: If the furnace still will not stay lit after these checks, the right next action is professional service. At that point you have ruled out the common homeowner fixes without getting into unsafe gas or control work.
What to conclude: A persistent shutdown after the safe checks usually means the furnace is protecting itself from a combustion, venting, ignition, or control problem that needs proper testing.
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That pattern often points to flame proving trouble, especially a dirty furnace flame sensor. It can also happen from a loose blower door, control interruption, or a deeper combustion fault. Start with the filter, panel fit, and flame-sensor check before assuming major parts are bad.
Yes. A badly restricted furnace filter can cause overheating and make the burners shut off early on a limit safety. That usually shows up as the furnace running a little longer than a flame-sensor problem, but a filter is common enough that it is always worth checking first.
Usually, yes, if you shut off power first, can clearly identify the sensor, and only remove and lightly clean the metal rod. If you are unsure which part you are looking at, or the burner area shows soot, water, or damaged wiring, stop and call for service.
No, not as a first guess. Pressure-switch faults are often caused by vent blockage, condensate issues, weak draft, or wiring problems. On this symptom, a pressure switch is not a good blind-buy part for homeowners.
Call for service if you smell gas, hear delayed ignition, see yellow or unstable flames, find water in the furnace, get repeated lockouts, or the furnace still will not stay lit after checking the thermostat, filter, blower door, and flame sensor. Those remaining causes need proper combustion and electrical testing.