Timing tells you a lot
| Shuts off after | Most likely |
|---|---|
| 3–7 seconds of flame | Flame sensor cannot prove flame |
| A few minutes of good heat | High-limit trip from restricted airflow |
| Before ignition, after the inducer runs | Pressure switch not closing — blocked vent or condensate backup |
| Randomly, at no consistent point | Loose wiring, failing control board, or intermittent thermostat connection |
| Whenever the blower ramps up | Failing capacitor or motor drawing excess current |
The most common chain of events
A dirty filter restricts airflow. Less air across the heat exchanger means the exchanger gets hotter than designed. The high-limit switch opens to protect it, cutting the burners while leaving the blower running to remove residual heat. Once things cool, the limit resets and the furnace tries again. You experience this as short bursts of heat and a house that never reaches setpoint.
Repeated limit trips are hard on a heat exchanger. This is the main reason we push filters as strongly as we do — it is a $15 item preventing a very expensive one.
Safe checks
- Replace the filter, using the size printed on the filter slot or the old filter's frame.
- Open all supply registers and clear furniture or rugs away from return grilles.
- Confirm the outdoor PVC intake and exhaust are clear of snow and ice.
- Check that the condensate drain or pump is not full, clogged, or frozen.
- Time the shutdown with a phone stopwatch — that number genuinely helps diagnosis.
- Note the LED flash code inside the furnace viewport.
Where safe checks end
Never bypass a safety switch, open or adjust a gas valve, disassemble burners or the heat exchanger, jump control board terminals, or work on live wiring. If the safe checks below do not restore heat, stop and call 716-412-2677.
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Buffalo-specific triggers
Condensate is the underrated one. High-efficiency furnaces produce several gallons of water a day during heavy use, and in an unheated or poorly insulated Buffalo basement that drain line — or the pump discharge tubing running to a laundry tub or outside — can freeze. When water backs up, the pressure switch opens and the furnace shuts down mid-sequence. Homeowners read it as a random failure; it is actually a very local, very seasonal problem.