Lennox Furnace Error Code SIMULTANEOUS FAST FLASH

The simultaneous fast flash pattern on a Lennox furnace control board indicates normal operation. It signals that the thermostat has initiated a heating demand and the control board has received the call for heat. This is a status indicator, not a fault code, and no corrective action is needed when this pattern appears during normal furnace operation.

No, this pattern is not a safety concern. It is a routine status indicator built into the Lennox control board diagnostic system to communicate that a heating demand is active. If you observe this pattern and the furnace is operating normally with heat being produced, the system is functioning as designed.

A diagnostic service call to identify why a furnace is not heating despite receiving a thermostat demand typically costs between $75 and $150 for the visit alone, with additional costs depending on what component has failed. Common components found at fault in these situations, such as igniters, flame sensors, or pressure switches, generally range from $20 to $200 or more in parts, plus labor.

No repair is needed for this indication on its own, since it represents normal furnace operation during a heating call. However, if your furnace is displaying this pattern but failing to produce heat, you should investigate downstream components such as the igniter, gas valve, flame sensor, or pressure switches. The issue in that scenario would not be the signal itself but rather a failure in the ignition or heating sequence that follows.

Resetting the furnace will temporarily clear the pattern, but it will reappear as soon as the thermostat sends another heating demand, which is expected behavior. Because this is not a fault code, there is nothing to clear in the traditional sense. If you reset the furnace and the pattern does not return even when the thermostat is calling for heat, that could indicate a wiring or thermostat communication issue worth investigating.