Lennox Heat Pump Frozen and Not Defrosting: Fixes & Cost

Last updated Oct 6, 2026 · By fixme.vip Editorial

Lennox heat pump frozen and not defrosting: what it means

“Frozen and not defrosting” is a symptom, not a numbered fault code. It means the outdoor coil of your Lennox heat pump has iced over and the normal defrost cycle is not clearing it.

In heating mode the outdoor coil runs below freezing, so frost naturally forms on it. The defrost control handles this in four steps:

• It reads a coil sensor and, on demand-defrost boards, an outdoor ambient sensor.
• When it detects frost, it shifts the reversing valve into cooling mode for a few minutes.
• At the same time it stops the outdoor fan, which lets hot refrigerant melt the ice.
• It usually brings on auxiliary heat indoors so you don’t feel cold air during the cycle.

If any part of that chain fails, ice builds cycle after cycle. Eventually it can encase the coil, block airflow, damage fan blades, and stress the compressor.

How the fault is reported depends on the model:

• Communicating models (XP25, SL25XPV and similar iComfort/S30/S40 systems) may post a defrost or sensor alarm on the thermostat.
• Conventional models usually show nothing indoors. Only the LEDs on the outdoor defrost board indicate a problem.

Many Lennox demand defrost boards also lock out after detecting repeated sensor or pressure faults. Some clear when power is cycled, and others need the fault corrected first. This varies by board revision. Cycle power once at most. If the unit freezes again, it needs a technician.

Lennox heat pump frozen and not defrosting: common causes, ranked by probability

Lennox does not publish a ranked troubleshooting list for this symptom. The order below is editorial and reflects common field experience. It is not a manufacturer ranking.

1. Water dripping or pooling on the unit. Water from a gutter or roof edge can drip onto the unit and refreeze faster than defrost can clear it. Drifted snow or a low pad that floods and freezes has the same effect. The defrost system itself may be working fine.
2. Failed or misplaced defrost coil sensor. If the sensor reads the coil as warmer than it is, or has come loose from its tube, the board never starts a defrost or ends it too early.
3. Defrost control board failure. A failed relay, a bad output to the reversing valve, or a board stuck in a lockout state prevents defrost from starting. A wrong defrost interval setting on time-temperature boards can also cause it.
4. Reversing valve or solenoid coil failure. The board calls for defrost, but the valve doesn’t shift, so the coil never gets hot gas.
5. Low refrigerant charge (leak). Undercharge drives coil temperature down and causes heavy, uneven icing, often worst on the lower part of the coil.
6. Outdoor fan not stopping during defrost. This is usually a board relay or wiring fault. The fan blows cold air across the coil and the ice never melts.
7. Restricted indoor airflow. In heating mode the indoor coil acts as the condenser. A clogged filter, closed registers or a weak blower therefore raises head pressure rather than lowering it. The connection to outdoor icing is indirect. If pressure climbs high enough to trip the high-pressure switch, the board can lock out, and a locked-out board won’t run defrost.
8. Dirty or damaged outdoor coil. Dirt and bent fins trap frost and slow melting. This mostly makes other causes worse rather than acting alone.

How to fix Lennox heat pump frozen and not defrosting: safe checks first

• Switch to Emergency Heat (or Aux Heat). This shuts off the heat pump and protects the compressor while your backup heat runs. Leave it that way until the ice has melted and the unit is serviced.
• Replace or check the air filter. A clogged filter raises system pressure in heating mode and can trip the high-pressure safety. That can lock out the board and stop defrost. Install the correct size with the airflow arrow pointing toward the blower.
• Open supply registers and clear return grilles. Make sure furniture, rugs or curtains aren’t blocking them.
• Check thermostat settings and batteries. Confirm the thermostat is set to Heat with a reasonable setpoint. Replace the batteries if the display is dim or blank.
• Check the breakers. Confirm both the outdoor unit breaker and the indoor air handler or furnace breaker are on. If a breaker has tripped, reset it once. If it trips again, stop and call a technician.
• Look for dripping water and drifted snow. Water from a gutter or roof edge landing on the unit is a common culprit, so note it for repair. Gently brush loose snow away from the sides and top by hand. Never chip or pry at ice on the coil.
• Make sure the outdoor access panels are in place and secure.
• Cycle power once. Wait until the ice has fully melted. Then turn the outdoor disconnect or breaker off for a minute and back on to clear a possible board lockout. If the coil ices up again, don’t keep resetting it. Call a technician.

How a technician will diagnose it

Use this list to sanity-check a quote. A thorough technician will typically:

1. Thaw the coil and inspect it. They look at the icing pattern. Ice only at the bottom suggests low charge or poor drainage. A fully encased coil suggests defrost never started.
2. Read the defrost board LEDs or thermostat alarms. They compare what they see with the code chart on the unit’s wiring diagram, since codes vary by board revision.
3. Check the defrost coil sensor. They confirm it is mounted correctly, then measure its resistance at a known temperature against Lennox’s temperature-resistance chart.
4. Force a test defrost. They use the board’s TEST pins or the thermostat’s service menu. Then they confirm three things: the reversing valve shifts, the outdoor fan stops, and auxiliary heat comes on.
5. Test the reversing valve. They check for 24 VAC at the solenoid during defrost and measure the solenoid coil. Temperature readings across the valve help them tell a bad coil from a stuck valve.
6. Check refrigerant charge and pressures. They connect gauges and measure superheat and subcooling. If the charge is low, they should find and repair the leak, not just add refrigerant.
7. Check indoor airflow. They measure static pressure and check blower operation.
8. Check the outdoor fan. They test the fan motor, its capacitor and the board’s fan relay.

Symptom, cause and what to do

Lennox heat pump frozen and not defrosting repair cost

Typical US ranges, including parts and labor:

• Filter, thermostat batteries, clearing snow: $0–$40 DIY
• Service call and diagnosis: $100–$250
• Defrost coil sensor: $150–$350
• Defrost control board: $300–$1,000+. Boards for communicating or variable-capacity models are at the higher end.
• Reversing valve solenoid coil: $150–$400
• Reversing valve replacement: $1,000–$2,500+. This is a brazed refrigerant repair with evacuation and recharge.
• Refrigerant leak repair and recharge: $250–$1,500+, depending on leak location and refrigerant type (R-410A or R-454B).
• Outdoor fan motor and capacitor: $300–$700
• Outdoor coil cleaning: $100–$250
• Riser stand or drainage correction: $150–$500

Lennox’s limited warranty may cover many parts. Labor usually is not covered. Warranty terms vary by product line. Registration typically determines the length of coverage, because registered units often qualify for a longer, extended parts warranty than unregistered ones. Check your product line’s warranty terms and your registration status before approving major repairs.

Related codes

• Lennox Heat Pump Not Heating: Causes, Fixes & Costs
• Lennox Heat Pump Not Cooling: Causes, Fixes & Costs (reversing valve and charge problems overlap)
• Lennox Furnace Code E180: Outdoor Sensor Fix & Cost (for dual-fuel systems)
• Lennox Furnace Blower Not Running: Causes & Fixes (indoor airflow problems that can raise head pressure and trip a high-pressure lockout)