Lennox · Furnace / HVAC
4 blinks Limit switch open (overheating)
What this code means
The furnace exceeded its safe temperature limit and the high-limit switch cut the burners.
Most likely causes
- Dirty air filter restricting airflow
- Closed supply vents throughout the house
- Failing blower motor
What to try first
Replace the air filter and make sure supply vents aren't closed off room to room.
Good news: this one is often fixable without a service call. If it persists after the steps above, have it looked at.
Find the part on Amazon →FAQ
What does Lennox error code 4 blinks mean?
Limit switch open (overheating). The furnace exceeded its safe temperature limit and the high-limit switch cut the burners.
Can I fix Lennox code 4 blinks myself?
Often, yes. Replace the air filter and make sure supply vents aren't closed off room to room.
What causes Lennox code 4 blinks?
The most common causes are: dirty air filter restricting airflow; closed supply vents throughout the house; failing blower motor.
Other Lennox furnace codes
All Lennox furnace error codes →
Other furnace brands: Trane · Carrier · Goodman · Amana · American Standard · Rheem · Bryant · Generic · Nest
This information may contain errors or not apply to your exact model — always double-check it against your appliance's own manual before acting. If there's any chance of a gas, fire, electrical, carbon monoxide, or other safety hazard, stop and call a licensed professional or the appropriate agency (e.g. your gas utility) instead of relying on this page alone.