HVACopediaLennox · Heat pump
Green & Red alternating FAST flash Coil (defrost) sensor problem
What the control is telling you
The outdoor coil sensor reads open, shorted or out of range. The control will NOT run demand or time/temperature defrost, so the outdoor coil can ice up in heating; the system still heats and cools.
Applies to: Lennox EL16KP1 (Elite) and ML14KP1 (Merit) R-454B heat pumps, sizes -018 to -060, demand defrost control A108 (CMC1)
Likely causes
- Coil sensor open or shorted
- Coil sensor unplugged or loose at P4 (DIS-YEL)
- Sensor off its mounting location
- Damaged sensor wiring
Test and repair steps
- Check the coil sensor plug at P4 (P4-5 ground, P4-6 signal) and its wiring.
- Make sure the sensor is clipped on the tube between the TXV outlet and the distributor (location matters for defrost).
- Unplug the sensor and check it with an ohmmeter for open or short; replace if faulty.
- If the coil is iced, clear it by forcing a defrost with the TEST pins once the sensor is fixed.
- Cycle 24 VAC power and confirm the code clears.
Settings for these units
Outdoor unit clearances · Lennox EL16KP1
| Location | Clearance |
|---|---|
| Service side (one side next to control box) | 30 in. (762 mm) |
| One of the other three sides | 36 in. (914 mm) |
| One of the remaining two sides | 12 in. (305 mm) |
| Last side | 6 in. (152 mm) |
| Between two units | 24 in. (610 mm) |
| Top | 48 in. (1219 mm) |
| Above roof surface (roof mount) | 6 in. (152 mm) minimum |
Increase clearances where snow/ice can fall on the unit or air can recirculate (decks, tight spaces).
Wind barrier, if used, 24 in. (610 mm) from the unit.
Outdoor unit clearances · Lennox ML14KP1
| Location | Clearance |
|---|---|
| Service side (one side next to control box) | 24 in. (610 mm) |
| One of the other three sides | 36 in. (914 mm) |
| One of the remaining two sides | 12 in. (305 mm) |
| Last side | 6 in. (152 mm) |
| Top | 48 in. (1219 mm) |
| Multi-unit, between units | 12 in. (305 mm) minimum [18 in. (457 mm) recommended] or 24 in. (610 mm) depending on layout |
| Multi-unit service side | 24 in. (610 mm) recommended |
| Above roof surface (roof mount) | 6 in. (152 mm) minimum |
Multi-unit spacing (A/B) depends on single row, wall or multiple rows layout - see figure on p.7.
Increase clearances where snow/ice can fall on the unit or air can recirculate.
Models that show this code
14 models in Lennox EL16KP1, Lennox ML14KP1
Other codes on the same units
- Green & Red alternating SLOW flash 5-minute anti-short-cycle delay
- Green & Red simultaneous FAST flash Ambient (outdoor air) sensor problem
- Green FAST flash / Red OFF Discharge sensor lockout
- Green FAST flash / Red ON Discharge line temperature lockout
- Green OFF / Red FAST flash Discharge sensor fault
- Green OFF / Red OFF No power to the demand defrost control
- Green OFF / Red ON Low pressure lockout
- Green OFF / Red SLOW flash Low pressure fault
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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.
Homeowner looking at this code? See what it means for you and when to call a technician.
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.