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Lennox · Heat pump

Green SLOW flash / Red OFF High pressure fault

Medium severity Lennox EL16KP1Lennox ML14KP1

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

  1. Clean the outdoor coil and make sure the outdoor fan runs at the right speed and current.
  2. In heating, check indoor airflow (dirty filter, closed registers, blower speed) - the indoor coil is the condenser.
  3. Check the charge with the subcooling method; an overcharge raises head pressure.
  4. Compare operating pressures with the Normal Operating Pressures table for the unit size.
  5. Confirm the correct indoor expansion valve is installed and working.
  6. 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.

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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.