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Lennox · Heat pump

Green & Red simultaneous FAST flash Ambient (outdoor air) sensor problem

Medium severity Lennox EL16KP1Lennox ML14KP1

What the control is telling you

The outdoor ambient sensor reads open, shorted or out of range. The control falls back to time/temperature defrost; 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

  • Ambient sensor open or shorted
  • Ambient sensor unplugged or loose at P4 (AMB-BLACK)
  • Damaged sensor wiring

Test and repair steps

  1. Check the ambient sensor plug at P4 (P4-3 ground, P4-4 signal) and its wiring.
  2. Unplug the sensor and check it with an ohmmeter for open or short; replace if faulty.
  3. Cycle 24 VAC power and confirm the code clears.
  4. Until repaired, defrost runs on time/temperature only.

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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.