Lennox Error Code Fast Flash / Slow Flash

Full Diagnosis

Reversed Polarity / Grounding Fault. The DS1/DS2 board continuously monitors incoming 120V hot-neutral-ground wiring, and this pattern means it has detected either reversed line polarity (hot and neutral swapped) or an inadequate equipment ground. The board deliberately locks out and won't energize the inducer or igniter, since flame sensing depends on a solid ground reference and can behave unpredictably with bad polarity or grounding. In the field this shows up as the inducer never attempting to start at all, with the fault persisting until the wiring is corrected and power is cycled.

All Recorded Causes

Hot and neutral reversed at the line-voltage plug or disconnect, missing or corroded ground wire at the furnace's ground lug, paint or rust under the ground screw preventing metal-to-metal contact, a shared ground connection that's loose or daisy-chained, or temporary power (extension cord/generator) wired without a proper ground. Check orientation and continuity with a plug-in circuit tester or meter from the cabinet ground lug back to the panel ground before replacing the board.

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 hot and neutral reversed at the line-voltage plug or disconnect, missing or corroded groun….
3. Verify each safety device and prove where the sequence stops under live operation, including whether check orientation and continuity with a plug-in circuit tester or meter from the cabinet g….
4. After the correction, run the system long enough to confirm the ds1 & ds2 fault does not return under normal load.