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

This code indicates a coil sensor problem. The control board has detected that the coil temperature sensor connected via brown wires at pins 5 and 6 is either open, shorted, or reading outside the expected temperature range. As a result, the control board disables all defrost operation, both demand-based and time and temperature defrost, until the fault is resolved.

Running the system with this fault active is not recommended for extended periods. Because defrost operation is completely disabled when this code is present, ice will accumulate on the outdoor coil during heating mode. Heavy ice buildup restricts airflow, reduces efficiency significantly, and can place strain on the compressor over time. Addressing the fault promptly prevents secondary damage.

Repair costs vary depending on whether the issue is the sensor itself, the wiring harness, or the control board. Sensor and harness replacements are generally the least expensive repairs, typically falling in the range of $75 to $250 in parts and labor combined. If the control board input circuit is at fault, costs can be higher, often ranging from $300 to $600 or more depending on the board and labor rates in your area.

A knowledgeable DIYer can begin by visually inspecting the brown wire harness running to the coil sensor for obvious damage, corrosion, or loose connections at pins 5 and 6. Measuring resistance across those pins requires a multimeter and some familiarity with sensor diagnostics. Replacing the sensor harness is a straightforward repair if the harness is confirmed faulty, but working inside the unit involves electrical components, so comfort with basic electrical safety practices is important.

A power reset may temporarily clear the code, but it will return quickly if the underlying sensor or wiring fault still exists. The control board continuously monitors the coil sensor signal during operation, so any persistent open, short, or out-of-range condition will retrigger the alternating fast flash pattern. A lasting fix requires diagnosing and repairing the sensor circuit.