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Error code reference
Lennox Furnace Error Code SIMULTANEOUS SLOW FLASH
Power on
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What the SIMULTANEOUS SLOW FLASH error code actually means
The flashing lights on your Lennox furnace are not a warning or error in this case. They simply mean your furnace is on and working normally, either cooling your home, running the fan, or waiting for instructions from your thermostat. No action is needed.
Diagnostic procedure
The full SIMULTANEOUS SLOW FLASH procedure
Step 1 is visible so you can confirm you are on the right page. The remaining steps, the ones that tell you whether to actually replace a part, unlock with any pass.
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1
Confirm the fault
Power cycle the unit: switch off the breaker for 60 seconds, then restore power. If the code returns, the fault is persistent and the procedure below isolates the cause.
More steps, wiring diagram references, and verified part numbers
Includes expected resistance and voltage values, connector pinouts, and supersession notes for replacement parts.
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Most common causes of SIMULTANEOUS SLOW FLASH
Ranked by how often they cause this code
- 1Normal standby or powered-on state
- 2Active cooling operation
- 3Continuous fan mode
- 4Thermostat not sending a heat call
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Questions people ask about SIMULTANEOUS SLOW FLASH
What does SIMULTANEOUS SLOW FLASH mean on a Lennox furnace?
The simultaneous slow flash on a Lennox furnace indicates normal operation. It is displayed by the control board LED indicators during powered standby, active cooling cycles, and continuous fan operation. It is not a fault code and does not indicate any problem with the furnace.
Is it safe to run my Lennox furnace when it is showing this flash pattern?
Yes, this flash pattern confirms safe, normal operation. The control board uses different LED patterns to indicate fault conditions that may require shutting the system down. The simultaneous slow flash specifically means no fault has been detected.
Will resetting my Lennox furnace clear the simultaneous slow flash?
Resetting the furnace will not change this signal because it is not an error code to be cleared. After a reset, the control board will return to its powered standby state and display the same simultaneous slow flash again, confirming normal operation has resumed.
Do I need to fix anything when I see the simultaneous slow flash on my Lennox furnace?
No repair is needed when this signal is displayed. It confirms the control board is powered and the system is operating normally. If the system is not heating or cooling as expected despite showing this status, the issue likely lies with thermostat settings or wiring rather than a furnace fault.
How much does it cost to diagnose or repair a simultaneous slow flash on a Lennox furnace?
Because this is a normal status indication and not a fault, there is no repair associated with it and no parts cost. If a technician visit is needed to confirm system operation or to investigate why heating is not occurring despite the normal status signal, diagnostic service calls typically range from $75 to $150 depending on your region and provider.
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