System Diagnosis
Flame Failure / Watchguard Mode. The system has failed to ignite or lost flame signal multiple times.
Lennox DS1 & DS2
Error Code Alt Slow Flash / Alt Slow Flash
Troubleshooting guide for the Lennox DS1 & DS2 system fault Alt Slow Flash / Alt Slow Flash.
ACLogics Fix Lennox code Alt Slow Flash / Alt Slow Flash See how ACLogics handles it → Fix Alt Slow Flash / Alt Slow Flash step-by-step
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.
Potential Causes & Solutions
- Dirty flame sensor, gas valve issue, igniter failure, gas supply issue.
Verify Before Replacing Parts
- Confirm the exact Lennox model family and ds1 & ds2 context before acting on a generic code description.
- Walk the entire heating sequence in order instead of jumping straight to the reported code.
- Verify each safety device and prove where the sequence stops under live operation.
- After the correction, run the system long enough to confirm the code does not return under normal load.
Field Signals to Confirm
What usually confirms it
A repeatable stop in the heating sequence at the same safety or prove step.
What can fool you
Weak grounding, marginal flame signal, or venting issues can produce inconsistent code behavior.
When to Escalate the Call
- Safety devices keep opening after the obvious mechanical causes are corrected.
- Combustion, venting, or gas input needs a higher-confidence review.
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.
