Carrier Heat Pump Frozen and Not Defrosting: Causes & Fixes

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

Carrier 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 Carrier heat pump has built up ice and the defrost cycle isn’t removing it.

In heating mode, the outdoor coil runs below freezing, so frost forms. To clear it, the defrost control briefly reverses the system into cooling mode, which sends hot refrigerant through the outdoor coil. During this cycle the outdoor fan stops and the backup heat usually turns on.

How defrost is triggered depends on the board:

• Time-temperature boards (common on older and standard Carrier units) start defrost at set run-time intervals, typically 30, 60 or 90 minutes. Some boards use 30, 50 or 90 minutes instead. Defrost only starts when the coil sensor reads cold, around 30–32°F or below. The exact intervals and temperatures vary by board, so check the wiring diagram on your outdoor unit.
• Demand-defrost and communicating boards (Infinity, Greenspeed and some Performance models) use temperature and other inputs to decide when to defrost.

The board ends defrost when the coil sensor warms to its termination temperature, or when a time limit is reached.

When this system fails, ice keeps building until the coil is solid. Airflow through the coil stops, and heat output collapses. This is not a self-resetting condition. A board fault may clear after you cycle power, but the underlying cause usually returns. Treat this as a technician repair.

If your heat pump runs but the house is cold without visible ice, see Carrier Heat Pump Not Heating instead.

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

These causes are ranked roughly by how often they explain a Carrier outdoor coil that ices over. Conditions around the unit come first, because they are common and easy to rule out. Defrost controls and the refrigerant system come next.

1. Restricted airflow across the outdoor coil. Snow drifts, leaves, a dirty coil, or a unit sitting too low can cause heavy icing that a normal defrost can’t keep up with.
2. Water dripping onto the unit or poor drainage. A missing gutter, roof runoff, or a unit sitting in a puddle can encase the base and coil in ice that defrost can’t melt.
3. Failed or misplaced defrost sensor/thermostat. The coil sensor is clipped near the bottom of the outdoor coil. If it fails, comes loose, or reads warm, the board never sees “cold enough” and never starts defrost.
4. Faulty defrost control board. A failed relay, timer circuit or board can keep the unit from switching into defrost. It can also keep the outdoor fan from shutting off during defrost.
5. Low refrigerant charge (leak). Low charge makes the outdoor coil run abnormally cold. It can also keep defrost from fully clearing the coil. Ice often forms unevenly, starting at the bottom or in one section.
6. Reversing valve or solenoid coil failure. If the valve or its 24V solenoid doesn’t shift, the board may enter defrost but the coil never gets hot gas.
7. Outdoor fan motor or capacitor problems. A slow or failing outdoor fan can contribute to icing. This points a tech toward electrical diagnosis.
8. Indoor airflow problems. A clogged filter or weak blower raises system pressures and can affect defrost performance. These matter less for outdoor icing than the causes above, but they are easy to rule out.

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

• Switch to Emergency Heat. Set the thermostat to EM Heat, or turn the heat pump off if you have a furnace backup. This protects the compressor while you wait for service.
• Watch for a defrost cycle. In heat mode, a normal defrost lasts 2–10 minutes. You’ll see the outdoor fan stop, hear a “whoosh” as the valve shifts, and often see steam. If you watch the unit for 90 minutes or more in freezing weather and see no defrost, note that for the technician.
• Check the thermostat. Confirm it’s set to Heat with a sensible setpoint. Many Carrier thermostats use 2 AA or AAA batteries, and many are hardwired with no batteries at all. If yours uses batteries and the display is dim or blank, replace them.
• Replace or check the air filter. A clogged filter restricts indoor airflow.
• Clear around the outdoor unit. Use your hands or a broom to remove snow and leaves from around and on top of the unit. Keep about 2 feet of clearance. Do not chip ice off the coil.
• Check for dripping water. If a gutter or roof edge drips onto the unit, fix the gutter or redirect the runoff.
• Check indoor vents and registers. Make sure they are open and unblocked.
• Check breakers and the outdoor disconnect. Confirm both are on. You can cycle power once at the breaker for about 5 minutes to reset the control board. If the coil refreezes, stop and call a technician.
• Make sure the outdoor unit’s access panels are seated. Don’t open them.

How a technician will diagnose it

• Confirm the complaint. The tech will look at the ice pattern.
  • Even frost across the whole coil usually means normal frosting between cycles, or points to an airflow or defrost-timing problem.
  • Ice only on the bottom or on part of the coil suggests low charge or a sensor issue.
• Check the board. They’ll read any LED or fault codes on the outdoor board. On communicating systems, they’ll check the wall control’s fault history.
• Force a defrost. They’ll use the board’s test pins or test button. Many Carrier boards speed up timing when the test pins are jumpered. This checks whether the fan stops, the reversing valve shifts and the coil warms.
• Test the sensor. They’ll check the defrost sensor’s resistance or continuity against Carrier’s temperature chart. They’ll also confirm it’s clipped tightly in the correct location.
• Check board outputs. They’ll measure the 24V signal to the reversing valve solenoid. They’ll also check the relay that cuts the outdoor fan during defrost.
• Check refrigerant pressures and temperatures. They’ll measure superheat and subcooling. If the charge is low, they should leak-search before recharging.
• Inspect the outdoor fan and unit. They’ll check the fan motor, capacitor, coil cleanliness, and how high the unit sits off the ground.
• Expect a clear explanation. A good quote names the failed component and the test result that proved it. Be wary of a quote that says only “add refrigerant” without a leak check. Be equally wary of “replace board” without a forced-defrost test.

Symptom, cause and what to do

Carrier heat pump frozen and not defrosting repair cost

Typical US costs, including parts and labor:

Carrier’s limited warranty may cover parts, typically for 10 years with registration, but it often doesn’t cover labor. Check before you approve the work.

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