Lennox Furnace Keeps Shutting Off: Causes, Fixes & Costs

Last updated Sep 29, 2026 · By fixme.vip Editorial

Lennox furnace keeps shutting off: what it means

“Keeps shutting off” is a symptom, not a single Lennox fault code. It describes a furnace that starts a heating cycle — inducer, ignition, and burners — then shuts down before the thermostat is satisfied, often repeating the pattern. On Lennox boards this behavior is usually the furnace protecting itself: a safety switch has opened, or the control cannot prove flame, so it shuts the gas valve.

Lennox uses SureLight integrated ignition controls across several generations, and older non-SureLight boards may flash a light pattern rather than display a code. Depending on which control your furnace has, it may either auto-reset after a delay or hold in lockout until power is cycled. If it locks out, most Lennox controls require a power cycle or one thermostat reset to clear. Because the root cause can be an overheating heat exchanger, this is rated a pro repair once basic airflow checks are done.

Do not try to match a flash pattern or displayed code to a number you found online. Code meanings differ between board generations, so the only reliable reference is the diagnostic legend printed inside your furnace’s access panel or the installation manual for your specific unit.

If you smell gas, do not troubleshoot. Leave the house, avoid switches, and call your gas utility’s emergency line or 911 from outside.

Lennox furnace keeps shutting off: common causes, ranked by probability

1. Restricted airflow (dirty filter or blocked returns). The most common trigger. Reduced airflow overheats the heat exchanger, the high-limit switch opens, and the burners shut off.
2. Dirty or failing flame sensor. The board can’t confirm flame within a few seconds of ignition, so it closes the gas valve and shuts down — often mid-cycle.
3. Limit switch tripping. An open high-limit (from overheating or a failing switch) cuts the burners repeatedly.
4. Pressure switch opening during the cycle. A blocked flue/intake, weak inducer, or condensate backup opens the pressure switch mid-run.
5. Weak flame signal or ignition dropout. A marginal flame current causes the control to abort the cycle, sometimes after several retries.
6. Failing control board or gas valve. Less common; an intermittent board or valve relay can shut the burners without a clear code.
7. Thermostat issues. Weak batteries, loose wiring, or a mis-set thermostat can cut demand and mimic shutdowns.

How to fix Lennox furnace keeps shutting off: safe checks first

• Replace the air filter. A clogged filter is the #1 cause. Swap in a clean one of the correct size — check the size printed on the edge of your existing filter, since it varies by unit.
• Check thermostat settings and batteries. Confirm it’s set to Heat with a setpoint above room temperature; replace batteries if it’s battery-powered.
• Confirm the breaker and furnace switch. Make sure the furnace breaker is on and the switch on/near the unit (looks like a light switch) is on.
• Open and clear all supply and return registers. Blocked vents restrict airflow and cause overheating shutdowns.
• Check the condensate drain on high-efficiency models. A clogged or full condensate line can trip the pressure switch. Clear a visible blockage in the exterior drain line or empty the drain pan only — do not remove panels or disturb the pressure switch tubing.
• Reset once. Cycle power at the breaker one time. If the furnace fires and shuts off again, stop and call a technician — do not keep resetting.

How a technician will diagnose it

A qualified tech will:

• Read stored fault codes or flash patterns on the SureLight (or older non-SureLight) control and interpret them against the legend for that specific board.
• Measure temperature rise across the heat exchanger and check static pressure to confirm airflow.
• Inspect and clean or replace the flame sensor, then measure flame current in microamps.
• Test the high-limit and rollout switches for continuity and correct operation.
• Verify pressure switch operation, inducer draft, and flue/intake for blockage.
• Check the control board relays and gas valve signals with a multimeter.
• Inspect the heat exchanger for cracks or hot spots if overheating is suspected.

These steps involve opening the cabinet, live electrical testing, and gas-side work — all technician tasks.

Symptom, cause and what to do

Lennox furnace keeps shutting off repair cost

• Air filter: $10–$40 (DIY).
• Thermostat batteries: $5–$15 (DIY).
• Thermostat replacement: $120–$350 installed.
• Flame sensor clean: $80–$180. Replacement: $150–$300.
• Limit or rollout switch: $150–$350.
• Pressure switch: $150–$350.
• Inducer motor: $400–$1,200 installed.
• Control board: $400–$1,200 installed.
• Gas valve: $400–$900+ installed.
• Diagnostic/service call: $90–$180, often credited toward the repair.

Costs vary by region, model series, and parts availability. Get an itemized quote before authorizing board or gas-valve replacement.

Related pages

• Lennox Furnace Short Cycling: Causes, Fixes & Costs — closely related airflow and cycling issues.
• Lennox Furnace Not Igniting — when the burners never light at all.
• Lennox Furnace Blowing Cold Air — when the blower runs but the burners are off.

If your furnace is displaying a numeric code or a flash pattern, write it down and compare it against the diagnostic legend inside your furnace’s access panel or in the manual for your specific model. Because the same number can mean different things on different Lennox control generations, we don’t publish code-to-fault mappings unless they can be tied to a named manual and board.