System Diagnosis
Control Board Failure / Voltage Issue. The control board is not functioning properly or voltage is out of range.
Lennox DS1 & DS2
Error Code Alt Fast Flash / Alt Fast Flash
Troubleshooting guide for the Lennox DS1 & DS2 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
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
- 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.
- Inspect connectors, relay outputs, and any recently replaced electrical parts for mismatch or loose fit.
- 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
Missing voltage, inconsistent outputs, or a fault that changes when a connector is reseated.
What can fool you
Intermittent power quality and field wiring problems can mimic a failed board.
When to Escalate the Call
- You have unstable supply power, repeated fuse failures, or signs of board damage.
- The failure path involves multiple boards or field-modified wiring.
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.
