- Check the thermostat first — confirm it's set to HEAT with a setpoint above the current room temperature.
- A severely clogged air filter trips the high-limit switch, forcing blower-only mode to prevent overheating.
- Failed igniters and coated flame sensors are the most common mechanical causes — both are straightforward flat-rate repairs.
Blower runs but supply registers stay cool
If the blower cycles for several minutes with no warm air at the registers, the burner isn't igniting. The control board displays a fault code visible through the sight glass — photograph it before calling so your technician arrives with the right parts.
After changing a filter or resetting a breaker
High-limit switch trips often follow extended operation with a blocked filter. After replacing the filter, a full power cycle may clear the lockout. If it trips again within an hour, the limit switch itself warrants inspection.
What the igniter and flame sensor actually do
The hot-surface igniter glows to light the gas; the flame sensor confirms combustion and signals the control board to hold the gas valve open. A cracked igniter won't glow; a coated flame sensor reads flame as absent and shuts the valve within two seconds of ignition. Both are visible components a technician can test in minutes.
High-limit switch, airflow, and heat exchanger connection
Repeated high-limit trips accelerate heat exchanger fatigue. A furnace diagnosed with blower-only operation that hasn't been serviced recently warrants a heat exchanger inspection as part of the diagnostic visit — Legendary documents any finding in writing.
Cracked heat exchanger — a safety consideration
A cracked heat exchanger can allow combustion gases including carbon monoxide to enter the air stream. If a technician identifies a crack, continued operation is not advisable — Legendary documents any heat exchanger finding in writing and explains the options clearly.
Control board failure mimics igniter failure
Igniter replacements that don't resolve cold-air delivery often point to a failed control board. The board sequences igniter, gas valve, and blower — when it fails, symptoms can look like several different problems at once.
Questions homeowners ask
- Is it safe to keep running a furnace that blows cold air?
- Blower-only operation isn't immediately dangerous, but repeated ignition attempts stress the igniter and gas valve. If you smell gas or hear clicking with no ignition, shut the furnace off at the switch and call immediately.
- Can I clean a flame sensor myself?
- The procedure itself is simple, but it requires removing the furnace cover and working near gas components. Most Lake County homeowners prefer a flat-rate service visit that covers the diagnostic and the repair together.
- What does a flame sensor or igniter replacement cost?
- Legendary provides the price before any work begins — no after-the-fact surprises. The Free Second Opinion offer applies if you've already been quoted by another company.
Editorial provenance
Content by Legendary Service editorial team
Updated:
Methodology
Guidance compares the stated system factors, applicable utility program rules, and the conditions that must be confirmed in the home. Figures are planning ranges, not quotes; Legendary Service provides an upfront flat-rate price after diagnosis.