What It Means
Control board failure or out-of-range voltage issue.
Lennox All Systems
Lennox Error Code Alt Fast Flash / Alt Fast Flash
Troubleshooting guide for the Lennox system fault Alt Fast Flash / Alt Fast Flash.
ACLogics Fix Lennox code Alt Fast Flash / Alt Fast Flash See how ACLogics handles it → Fix Alt Fast Flash / Alt Fast Flash step-by-step
Possible Causes
- Bad control board.
- Voltage spike or drop.
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.
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Full Diagnosis
Control Board Failure / Voltage Issue. The control board is not functioning properly or voltage is out of range.
All Recorded Causes
Bad control board, voltage spike/drop.
Verify Before Replacing Parts
- Confirm the exact Lennox model family and ds1 & ds2 context before acting on a generic code description.
- Start with line voltage, low voltage, and board output checks in sequence so the fault is isolated cleanly — the data plate flags bad control board, voltage spike/drop.
- Inspect connectors, relay outputs, and any recently replaced electrical parts for mismatch or loose fit, and verify 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.
