Lennox Furnace Clicking But Not Starting: Causes & Fixes
Get this fixed by a local pro
Free, no-obligation quotes from licensed HVAC technicians in your area. Compare up to 3 estimates before anyone visits.
Lennox furnace clicking but not starting: what it means
“Clicking but not starting” is not a numbered Lennox fault code — it is a symptom. The clicking sound you hear is almost always a relay or the gas valve energizing and de-energizing on the furnace control. When the furnace tries to start, the control sequences the inducer, igniter and gas valve. If any step fails to prove out, the control drops the affected relay and may retry, producing repeated clicks with no burner ignition.
Lennox furnaces will attempt a set number of ignition tries and then hold in lockout until power is cycled or the thermostat is reset. The exact retry count, and whether the unit auto-resets after a period of time, varies by model and control.
Your furnace is also very likely storing an alert code that says which stage failed. Read the alert code on your board’s display or LED and look it up on the label inside the blower door, because code numbers and their meanings differ between Lennox SureLight ignition controls and the newer Lennox Unit Controller used on communicating equipment. On SureLight-type controls the code appears as an LED flash pattern; on the Unit Controller it appears as an alphanumeric code on the display. Do not assume a code number you found for one control platform means the same thing on the other — use the diagnostic label on your unit or the installation manual for your model.
Because the clicking traces back to the gas valve and control circuit, this is a pro-level diagnosis.
Lennox furnace clicking but not starting: common causes, ranked by probability
- Open or stuck pressure switch, blocked intake/exhaust, or condensate backup — the inducer runs and clicks in, but the pressure switch never proves, so the control halts before ignition. Blocked or iced intake/exhaust terminations and a backed-up condensate drain on high-efficiency models are the usual triggers. This is the single most common reason a Lennox furnace clicks and never lights.
- Weak or cracked hot surface igniter — the control energizes the igniter and gas valve, never proves flame, and drops out with a click on each retry.
- Dirty flame sensor / flame not proven — the burners may briefly light, but the sensor doesn’t confirm flame, so the gas valve relay drops out (a click) and the control retries, then locks out.
- Open limit or rollout safety — a tripped high limit (often from restricted airflow) or a rollout switch stops the sequence, and the control clicks as it cycles relays.
- Thermostat, low-voltage wiring, or weak transformer — an intermittent heat call, a loose low-voltage connection or insufficient 24 VAC makes relays chatter instead of holding in. (See Code E115, low secondary/24 VAC voltage.)
- Failed relay on the integrated control board — possible, but one of the least common causes. A control board should only be condemned after the safeties, igniter, flame sensor, venting and low-voltage supply have all been tested and found good.
How to fix Lennox furnace clicking but not starting: safe checks first
Stop first: if you smell gas, see scorching, soot or discoloration around the burner area, or notice a roaring or burning smell, shut the unit off, leave the house, and call the gas utility or 911 — do not troubleshoot.
Otherwise, these are the only steps safe for a homeowner. Do not open the sealed burner or gas compartment.
- Thermostat: Set it to Heat and a temperature several degrees above room temperature. Confirm it’s not in a hold or schedule that’s fighting you.
- Thermostat batteries: Replace them if your model uses batteries — a weak thermostat can drop the heat call intermittently.
- Air filter: A clogged filter restricts airflow and can trip the limit, interrupting the sequence. Replace it if dirty.
- Breaker / furnace switch: Reset the breaker once and confirm the wall switch by the furnace is ON.
- Registers and vents: Make sure supply and return vents are open and unblocked.
- Outside vent terminations: From outside the house, check that the intake and exhaust pipes are clear of snow, ice, leaves or nests.
- Condensate line (high-efficiency models): Check that the condensate drain isn’t clogged or backed up, which can trip a switch and stop startup.
- One reset only: If the unit is locked out, cycle the thermostat or breaker once. Do not repeatedly reset a furnace that keeps clicking and failing — that can be unsafe and won’t fix the root cause.
If it still only clicks after these checks, stop and call a licensed technician.
How a technician will diagnose it
A good tech will work the ignition sequence in order so you can sanity-check the quote:
- Read the stored alert code from the LED flash pattern or Unit Controller display and confirm its meaning against the diagnostic label on your specific unit.
- Verify line and 24 VAC control voltage at the control and transformer to rule out low-voltage relay chatter, and check low-voltage connections and thermostat wiring.
- Watch the ignition sequence live — inducer start, pressure switch closure, igniter glow, gas valve opening and flame proving — to see exactly which step fails.
- Test pressure switches and inspect venting for blockage, disconnected or cracked hoses, condensate backup and inducer performance.
- Ohm-test the hot surface igniter and inspect it for cracks or hot spots.
- Check the flame sensor microamp signal, then clean or replace as needed.
- Test the limit and rollout safeties for correct open/close operation, and look for the airflow or combustion problem that tripped them.
- Test the gas valve for proper energizing and gas supply pressure.
- Inspect the control board last for burnt relays or traces, only after the items above check out.
Ask which specific component tested bad and what reading it gave — a reputable tech can tell you. If the first thing you’re quoted is a control board, ask what was measured to rule out the safeties, igniter, flame sensor and venting.
Symptom, cause and what to do
| Symptom | Likely cause | DIY action | Technician job |
|---|---|---|---|
| Rapid clicking, no ignition attempt at all | Low or unstable 24 VAC, chattering thermostat, loose low-voltage wiring, or an open safety | Replace thermostat batteries; reset once | Test transformer and 24 VAC supply, tighten/repair low-voltage wiring, test safeties |
| Inducer runs, then clicks and stops | Open or stuck pressure switch, blocked intake/exhaust, condensate clog | Clear condensate line; check outside vent terminations | Test pressure switch and hoses, inspect inducer and venting |
| Clicks, igniter glows, no flame | Weak or cracked hot surface igniter | None safe | Ohm-test and replace igniter |
| Flame lights then clicks off after a few seconds | Flame not proven / dirty flame sensor | Replace filter | Clean or replace flame sensor, verify microamp signal |
| Clicks, then everything shuts down; blower may run | Open limit or rollout safety | Replace filter; open blocked vents; reset once | Test limit and rollout, find the airflow or combustion cause |
| Relay chatters, weak clicks, intermittent heat | Weak transformer or thermostat/wiring fault | Replace thermostat batteries; check breaker | Test transformer, inspect thermostat and low-voltage wiring |
| Everything else tests good, valve signal present but nothing holds | Failed relay on integrated control board (least common) | None safe | Confirm with voltage testing, then replace control |
Lennox furnace clicking but not starting repair cost
Honest US ranges including parts and labor. A diagnostic visit is typically $100–$250 and is often credited toward the repair.
- Air filter (DIY): $10–$40
- Thermostat batteries (DIY): $5–$15
- Flame sensor clean/replace: $80–$250
- Pressure switch: $120–$300
- Limit switch: $120–$300
- Control transformer: $120–$300
- Hot surface igniter: $150–$350
- Gas valve: $350–$900 installed
- Integrated control board: $400–$1,200 installed; communicating Lennox Unit Controller assemblies can run higher
If the furnace is well past 15 years and the control or gas valve has failed, ask whether repair is worth it versus replacement.
Related pages
- Lennox Furnace Code E115: Low 24 VAC Causes & Fixes
- Lennox Furnace Pressure Switch Problems: Causes, Fixes & Costs
- Lennox Furnace Flame Sensor Problems: Causes, Fixes & Costs
- Lennox Furnace Hot Surface Igniter Failure: Signs & Costs
- Lennox Furnace Limit and Rollout Trips: Causes & Fixes
- Lennox Furnace Won’t Ignite: Causes, Fixes & Costs
- Lennox Furnace Ignition Lockout: Causes, Fixes & Costs
- How to Read Lennox Furnace Alert Codes: SureLight Controls vs. the Lennox Unit Controller
Parts & tools for this fix
As an Amazon Associate, fixme.vip earns from qualifying purchases. Prices and availability are on Amazon.
Frequently asked questions
Why does my Lennox furnace click but not start?
The clicking is usually a relay on the control board or the gas valve energizing and dropping out as the furnace tries and fails to complete its ignition sequence. The most common reasons are an open or stuck pressure switch (often from a blocked intake/exhaust or a backed-up condensate drain), a weak or cracked hot surface igniter, a dirty flame sensor that never proves flame, or an open limit or rollout safety. A failed relay on the control board is possible but uncommon.
Is a clicking furnace dangerous?
A single click during a normal startup is fine. Stop and treat it as urgent if you smell gas at any point, if the furnace makes repeated ignition attempts that let unburned gas into the burners, or if you see scorching, soot or discoloration around the burner compartment. In those cases shut the furnace off at the wall switch or breaker, leave the house if you smell gas, and call the gas utility, 911 or a licensed technician. Otherwise, stop cycling the unit and have it inspected before it keeps locking out.
Can I fix the clicking myself?
You can safely check the thermostat, batteries, air filter, breaker, visible vents and the condensate line, and do one reset. Anything involving the gas valve, igniter, flame sensor, safeties, control board or internal wiring is technician work because it requires opening the cabinet and testing live voltage.
How much does it cost to fix a clicking Lennox furnace?
It ranges from $0 for a filter or thermostat fix to $80–$250 for flame sensor service, $150–$350 for an igniter, $350–$900 for a gas valve and $400–$1,200 or more for an integrated control board or communicating Unit Controller assembly, including labor. A diagnostic visit alone is usually $100–$250.