HVACopedia

Lennox · Heat pump

Green SLOW flash / Red ON Discharge line temperature fault

Low severity Lennox EL16KP1Lennox ML14KP1

What the control is telling you

Should not occur on these units: no discharge sensor is used, a 10K resistor sits on the discharge sensor terminals (COIL-BROWN). A shorted resistor makes the control see high discharge temperature. (The table text says ML14KP1 even in the EL16KP1 manual.)

Applies to: Lennox EL16KP1 (Elite) and ML14KP1 (Merit) R-454B heat pumps, sizes -018 to -060, demand defrost control A108 (CMC1)

Likely causes

  • Shorted 10K resistor on the discharge sensor connection
  • Wiring short at the discharge sensor terminals

Test and repair steps

  1. Turn off power and locate the 10K resistor on the discharge sensor connection of the defrost control.
  2. Measure it with an ohmmeter; it should read about 10K ohms. Replace it if shorted.
  3. Check that the resistor leads are not touching each other or the board.
  4. Cycle 24 VAC power and confirm the code clears.

Settings for these units

Save it for the next call

Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.

Open the app

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.