HVACopediaLennox · Heat pump
Green SLOW flash / Red OFF High pressure fault
What the control is telling you
The high pressure switch (S4) opened (trips at 590 +/- 15 psig, auto reset) and the control stopped the compressor and counted one strike.
Applies to: Lennox EL16KP1 (Elite) and ML14KP1 (Merit) R-454B heat pumps, sizes -018 to -060, demand defrost control A108 (CMC1)
Likely causes
- Restricted airflow over the indoor or outdoor coil (dirty coil, filter, blocked fan)
- Wrong refrigerant charge
- Wrong metering device installed or expansion valve not working properly
- Pressure switch or sensor mounted wrong or loose connection
Test and repair steps
- Clean the outdoor coil and make sure the outdoor fan runs at the right speed and current.
- In heating, check indoor airflow (dirty filter, closed registers, blower speed) - the indoor coil is the condenser.
- Check the charge with the subcooling method; an overcharge raises head pressure.
- Compare operating pressures with the Normal Operating Pressures table for the unit size.
- Confirm the correct indoor expansion valve is installed and working.
- Check the high pressure switch (S4, opens at 590 +/- 15 psig) wiring at HS-PS; make sure pressure switches and sensors are tight on the system.
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 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
- Green ON / Red OFF High pressure lockout
- Green ON / Red ON Demand defrost control failure
- Green SLOW flash / Red ON Discharge line temperature 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.