Lennox Heat Pump Error Code DS2 Simultaneous FAST Flash / DS1 Simultaneous FAST Flash

This fault code indicates a problem with the ambient temperature sensor, which monitors outdoor air temperature for defrost control. The control board has detected that the sensor circuit is open, shorted, or returning a temperature value outside its accepted range. When this fault occurs, the system automatically switches to time-based defrost operation as a fallback, so the heat pump continues to run but without sensor-guided defrost timing.

Resetting the unit may temporarily clear the fault display, but the error will return if the underlying sensor or wiring problem has not been corrected. Because the ambient sensor directly affects defrost control, the board will re-detect the fault as soon as it polls the sensor input again. A reset is not a fix and the system will continue operating in reduced time-based defrost mode until the sensor issue is resolved.

Repair costs vary depending on whether the fault is in the sensor itself, the wiring harness, or the control board connector. A sensor or harness replacement is generally a lower-cost repair, with parts and labor typically ranging from around $75 to $250 depending on your region and service provider. If the control board's input circuit is found to be at fault, costs can be higher. Getting a diagnosis from a licensed HVAC technician will give you an accurate estimate for your specific situation.

Start by measuring resistance across pins 3 and 4 on the control board, which carry the black wires from the ambient sensor. An open or shorted reading confirms a failed sensor or damaged harness, either of which should be replaced. If the wiring and connections appear intact but the error persists, the sensor element itself may have drifted out of its calibrated range and will need replacement. A qualified HVAC technician can perform these resistance checks and swap the component safely.

The heat pump is designed to continue operating in a fallback defrost mode when this fault is present, so it is not an immediate safety hazard. However, running without a functional ambient sensor means defrost cycles are based on fixed time intervals rather than actual conditions, which can reduce system efficiency and potentially allow ice buildup on the outdoor coil over time. Having the sensor repaired promptly is advisable to avoid unnecessary wear and efficiency loss.