Lennox Heat Pump: Heat pump not defrosting / outdoor coil iced – Lennox EL16KP1 / ML14KP1

Updated October 13, 2026 · by Aurora Star Tech Inc.

Heat pump not defrosting / outdoor coil iced - Lennox EL16KP1 / ML14KP1 on a Lennox heat pump: Ice builds up on the outdoor coil in heating and the unit does not clear it, or defrost ends before the coil is clear.

Homeowner with a heat pump problem? Call (647) 657-2977 or book a service call — licensed technicians across Vaughan and the GTA.

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

Common causes of heat pump not defrosting / outdoor coil iced - lennox el16kp1 / ml14kp1

Coil sensor fault (alternating fast flash) - no defrost at all
Coil sensor not clipped in the right spot
Termination temperature set too low for a cold climate
Reversing valve or its 24 VAC output (O OUT) not working
Low charge or poor airflow making frost build faster

How to fix heat pump not defrosting / outdoor coil iced - lennox el16kp1 / ml14kp1: step by step

Read the defrost control LEDs first: alternating FAST flash = coil sensor problem (no defrost); simultaneous FAST flash = ambient sensor problem (time/temperature defrost only).
Check the coil sensor is clipped on the tube between the TXV outlet and the distributor and plugged into P4.
Know the timing: the control counts heating run time while the coil is below 37°F; the first target is 90 minutes, then it adjusts up or down by 30 minutes based on the last defrost length. Below 10°F outdoor it waits 360 minutes; it goes back to 90 minutes when outdoor rises above 15°F.
Defrost ends when the coil reaches the termination setting (P1: 50, 70, 90 or 100°F; factory 50°F; 90°F if no jumper) or after 14 minutes maximum.
In cold climates, move the P1 jumper to a higher termination temperature if the coil is not cleared during defrost.
Force a defrost by jumpering the TEST pins; in defrost the control energizes the reversing valve, stops the outdoor fan and sends 24 VAC to W1 (auxiliary heat).
If the unit does not change over, check 24 VAC at O OUT and the reversing valve solenoid.
Check charge (subcooling) and indoor airflow; two low pressure trips in a row in heating will also start a defrost.
Return the test jumper to the desired termination pins when finished.

Safety: Gas, electrical and refrigerant work must be done by a licensed technician. Turn off power and gas before servicing, never bypass a safety control, and always follow the manufacturer's manual for your exact model.

If the unit shows an error code, look it up: Lennox Heat Pump codes

Models

Lennox EL16KP1

EL16KP1-018 EL16KP1-024 EL16KP1-030 EL16KP1-036 EL16KP1-042 EL16KP1-048 EL16KP1-060

Lennox ML14KP1

ML14KP1-018 ML14KP1-024 ML14KP1-030 ML14KP1-036 ML14KP1-042 ML14KP1-048 ML14KP1-060

Source: Lennox EL16KP1 Unit Information 100195 / ML14KP1 Unit Information 100108, Defrost System pp.30-31, Table 3 p.29, Table 4 p.32, Operating Sequence p.37. Written in our own words as a summary; always check the original manual.

General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.