Lennox Heat Pump Error Code Simultaneous FAST Flash (Green/Red)

This flash pattern indicates the control board has detected a problem with the ambient temperature sensor. The board found that the sensor circuit is either open (no continuity) or shorted (near-zero resistance), both of which produce readings outside the normal temperature-to-resistance range. The system will continue to heat and cool, but it shifts to a time/temperature based defrost strategy instead of using live ambient temperature data.

Yes, the system is designed to continue heating and cooling with this fault present. The control board automatically falls back to a time/temperature defrost schedule, which keeps the unit operational. However, defrost cycle accuracy is reduced without ambient sensor input, so leaving the fault unresolved for extended periods can allow ice buildup or reduce efficiency, especially in cold weather.

A power reset may temporarily clear the fault code display, but if the ambient sensor or its harness is actually open or shorted, the board will detect the problem again and the flash pattern will return. The error will only be resolved permanently once the defective sensor wiring or thermistor element is repaired or replaced and the sensor resistance reads within the expected range per Table 4.

A knowledgeable DIYer can start by visually inspecting the ambient sensor harness for obvious damage, corrosion, or loose connectors. Measuring sensor resistance with a multimeter and comparing it against Table 4 resistance values is the proper diagnostic step. However, if the harness or sensor needs replacement, comfort with outdoor unit wiring is required. If you are unsure, a qualified HVAC technician should perform the repair.

Repair cost depends on whether the fault is in the harness or the sensor element itself. Sensor harness replacement typically runs in the range of $75 to $200 in parts, plus labor. If only a connector cleaning or reseating is needed, costs are minimal. A service call to diagnose and repair the issue generally falls between $100 and $350 depending on labor rates in your area and what parts are required.