Lennox Heat Pump Error Code DS2:SimultaneousFastFlash / DS1:SimultaneousFastFlash

This code means the control board has detected a problem with the ambient temperature sensor on the outdoor unit. The sensor has been identified as open, shorted, or reading a temperature value outside the acceptable range. The system responds by reverting to a fixed time/temperature defrost schedule instead of demand-based defrost. Both diagnostic LEDs flashing simultaneously at a fast rate is the visual indicator for this specific fault.

Resetting the unit by cycling power may temporarily clear the displayed fault, but the error will return as soon as the board attempts to read the ambient sensor again and receives an invalid signal. A reset does not correct the underlying wiring or sensor problem. Proper diagnosis and repair of the sensor circuit are required to permanently clear this fault.

Repair costs vary depending on whether the issue is limited to the sensor harness, the thermistor itself, or the control board. A sensor or harness replacement is typically the least expensive repair, with parts and labor generally ranging from $75 to $250 depending on your location and service provider. If the control board requires replacement, costs can range from $300 to $700 or more. Getting a diagnosis from a qualified HVAC technician before purchasing parts is recommended to avoid unnecessary expenses.

Start by measuring resistance at pins 3 and 4 on the control board where the Black ambient sensor wire connects. Disconnect the harness and test resistance across the sensor leads: an open or near-zero reading points to a failed sensor or damaged harness. Inspect the harness along its entire length for chafing, pinching, or corroded terminals. Replace the sensor harness if shorted or open, or replace the ambient thermistor if the harness tests good but the sensor does not.

The unit will continue to operate with this error present because the control board automatically switches to a time/temperature defrost mode as a fallback. However, running in this mode reduces efficiency and may lead to inadequate or overly frequent defrost cycles depending on actual outdoor conditions. It is advisable to schedule a repair promptly to restore proper defrost control and protect the compressor from operating with excessive ice buildup on the outdoor coil.