HVACopediaLennox · Heat pump
Green OFF / Red FAST flash Discharge sensor fault
What the control is telling you
The control sees the discharge sensor input open. These units use a 10K resistor in place of a discharge sensor, so the resistor is open or disconnected. If the reading is not in range after 90 seconds the control counts one fault.
Applies to: Lennox EL16KP1 (Elite) and ML14KP1 (Merit) R-454B heat pumps, sizes -018 to -060, demand defrost control A108 (CMC1)
Likely causes
- Open 10K resistor on the discharge sensor connection
- Resistor disconnected or loose
Test and repair steps
- Turn off power and check the 10K resistor on the discharge sensor connection; it should read about 10K ohms.
- Replace the resistor if it reads open; reseat it if loose.
- Cycle 24 VAC power and confirm the code clears.
Settings for these units
- Outdoor unit clearances · Lennox EL16KP1
- Outdoor unit clearances · Lennox ML14KP1
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Source: Lennox EL16KP1 Unit Information 100195 / ML14KP1 Unit Information 100108, Table 4 Demand Defrost Control (A108) Diagnostic LEDs, p.32. Summarized in our own words; check the original manual for your model.
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For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.