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Error code reference

Lennox HVAC Error Code Slow Flash

Normal operation, no call for heat.

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What the Slow Flash error code actually means

The slow flashing light on your Lennox furnace means everything is working normally and the furnace is simply waiting for your thermostat to ask for heat. Your home has reached the temperature you set, so the furnace is resting. There is nothing wrong with the system.

Diagnostic procedure

The full 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.
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Most common causes of Slow Flash

Ranked by how often they cause this code
  1. 1Thermostat set point satisfied
  2. 2Thermostat in off or cooling mode
  3. 3Thermostat wiring issue
  4. 4Thermostat malfunction or dead batteries
  5. 5System recently completed a heating cycle

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Questions people ask about Slow Flash

What does Slow Flash mean on a Lennox hvac?
A slow flash on a Lennox furnace control board indicates normal standby operation with no active call for heat. The system is powered and functioning correctly, but the thermostat is not currently requesting heat. This is a status indicator, not a fault or error code. No service action is required when this is the expected system state.
Is a slow flash on a Lennox furnace a safety concern?
No, a slow flash is not a safety concern. It indicates the control board is operating normally with no faults detected. Safety-related issues on Lennox furnaces are communicated through different flash patterns, typically rapid flashes or specific fault code sequences.
How much does it cost to diagnose a Lennox furnace that shows a slow flash but will not heat?
A service call to diagnose a no-heat condition typically costs between $75 and $150 for the diagnostic visit alone. If the issue is traced to a faulty thermostat, replacement costs generally range from $25 to $300 depending on the thermostat model. Wiring repairs are usually included in the diagnostic or labor fee.
Do I need to fix anything if my Lennox furnace is showing a slow flash?
In most cases, no repair is needed. The slow flash simply means the furnace is in standby because the thermostat is satisfied. If the system is not heating when you expect it to be, check your thermostat settings, mode selection, and wiring connections before assuming there is a furnace problem.
Will resetting my Lennox furnace clear the slow flash?
Resetting the furnace is not necessary and will not change this indicator, since it is not a fault condition. The slow flash will continue as long as no heat call is present. It will automatically change to a different status once the thermostat sends a call for heat and the furnace begins a heating cycle.

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