Lennox Heat Pump Error Code Simultaneous FAST Flash

The Simultaneous FAST Flash code signals a problem with the ambient temperature sensor, which monitors outdoor air temperature to control defrost cycle timing. The control board has detected that the sensor circuit is open, shorted, or producing a reading outside the expected temperature range. The system will continue to heat and cool normally but switches to a basic time-and-temperature defrost schedule as a fallback until the sensor issue is resolved.

Yes, the system is designed to continue heating and cooling even with this fault active, reverting to a time-based defrost strategy as a backup. However, running long-term without a functioning ambient sensor can reduce efficiency and may not defrost the outdoor coil optimally in all weather conditions. It is best to have the issue diagnosed and repaired in a reasonable timeframe to avoid unnecessary wear.

A power reset may temporarily clear the fault code, but if the ambient sensor or its wiring is actually open, shorted, or producing out-of-range readings, the fault will return once the control board runs its diagnostic check again. The underlying cause must be identified and repaired at pins 3 and 4 of the control board for the error to clear permanently.

A knowledgeable DIYer can inspect the sensor wiring harness connected to pins 3 and 4 on the control board, identified by their black wires, for visible damage, loose connectors, or corrosion. If you have a multimeter, you can check for an open or shorted circuit across those pins. However, replacing control board components or sensor harnesses on a heat pump involves working inside electrical panels, so if you are not comfortable with that work, a licensed HVAC technician is recommended.

Repair costs depend on whether the issue is a simple wiring or connector problem or requires replacing the sensor and harness. A wiring repair or connector cleaning may cost relatively little in labor, while sensor and harness replacement typically runs in the range of $100 to $350 including parts and labor. Diagnostic service call fees vary by region and will generally add to the total cost.