Lennox Error Code Slow Flash / On

Full Diagnosis

Limit or Rollout Switch Open (Auxiliary Circuit). This alternate flash order — DS2 solid on while DS1 slow-flashes — flags the auxiliary safety circuit (typically the flame rollout or secondary limit switch) as open, distinct from the primary high-limit trip signaled by On / Slow Flash. When it trips, the board immediately shuts off the gas valve and igniter, the inducer keeps running to clear the heat exchanger, and the blower may continue running or ramp up to dump residual heat depending on the model. Once the switch cools and resets, the furnace will attempt to restart the heat cycle automatically.

All Recorded Causes

Dirty or clogged filter restricting return air, blocked or closed supply/return registers, undersized or crushed flex duct, wrong blower speed tap selected for the BTU input, blower wheel loose on its shaft or coated in dust, restricted flue/vent causing poor draft (common with rollout trips), or a failed/miswired limit or rollout switch — check static pressure and ohm the switch cold before replacing parts.

Verify Before Replacing Parts

1. Confirm the exact Lennox model family and ds1 & ds2 context before acting on a generic code description.
2. Walk the entire heating sequence in order instead of jumping straight to the reported code, watching for dirty or clogged filter restricting return air, blocked or closed supply/return registers,….
3. Verify each safety device and prove where the sequence stops under live operation, including whether the next candidate on the list.
4. After the correction, run the system long enough to confirm the ds1 & ds2 fault does not return under normal load.