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Lennox · Heat pump

Green OFF / Red SLOW flash Low pressure fault

Medium severity Lennox EL16KP1Lennox ML14KP1

What the control is telling you

The low pressure switch (S87, opens at 25 +/- 5 psig, closes at 40 +/- 5 psig) opened and the control stopped the compressor and counted one strike. It is ignored during defrost, for 90 s after defrost, for 90 s after compressor start in TEST mode, and for 10 minutes of compressor run when ambient is below 15°F. Two LP trips in a row in heating start a defrost.

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. Clear any blockage or restriction on the indoor and outdoor coils and fans; check the filter.
  2. Check indoor and outdoor fan motor current draw against the nameplate.
  3. Check the charge with the subcooling method and compare with the unit charging label / matchup table.
  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. Make sure the pressure switches and sensors are tight on the system so there are no leaks or false readings.

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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.