What It Means
Primary limit or rollout switch open due to furnace overheating.
Lennox All Systems
Lennox Error Code On / Slow Flash
Troubleshooting guide for the Lennox system fault On / Slow Flash.
ACLogics Fix Lennox code On / Slow Flash See how ACLogics handles it → Fix On / Slow Flash step-by-step
Possible Causes
- Dirty filter.
- Blocked vents.
- Bad blower motor.
- Restricted airflow.
Not sure which one it is — or if it's none of these? Try ACLogics today.
Disclaimer: This information is for general reference. Error codes can vary by model and production year. Always refer to your machine's specific manual and the information printed on the unit itself.
New to this code? Click to see why this happens
Full Diagnosis
Limit Switch Open. The furnace has overheated or the rollout switch has tripped.
All Recorded Causes
Dirty filter, blocked vents, bad blower motor, restricted airflow.
Verify Before Replacing Parts
- Confirm the exact Lennox model family and ds1 & ds2 context before acting on a generic code description.
- Confirm filter condition, blower operation, and coil cleanliness before replacing any controls — start by ruling out whether dirty filter, blocked vents, bad blower motor, restricted airflow.
- Measure airflow-related readings that match the symptom, such as temperature split or static pressure, and check whether the next candidate on the list.
- After the correction, run the system long enough to confirm the ds1 & ds2 fault does not return under normal load.
Not sure which system this Lennox code is on?
ACLogics has guided diagnostics for 16 Lennox fault codes and asks the right questions to narrow it down.
