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Lennox · Heat pump

Green & Red alternating FAST flash Coil (defrost) sensor problem

High severity Lennox EL16KP1Lennox ML14KP1

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

  1. Check the coil sensor plug at P4 (P4-5 ground, P4-6 signal) and its wiring.
  2. Make sure the sensor is clipped on the tube between the TXV outlet and the distributor (location matters for defrost).
  3. Unplug the sensor and check it with an ohmmeter for open or short; replace if faulty.
  4. If the coil is iced, clear it by forcing a defrost with the TEST pins once the sensor is fixed.
  5. Cycle 24 VAC power and confirm the code clears.

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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.