Heat pump defrost sensor: what it does and how it fails

30-second answer

Up to 10 minverified Sep 2026

A heat pump defrost sensor is a temperature sensor on the outdoor unit that the control board reads while the unit heats. Mitsubishi lists a defrost thermistor, and MRCOOL lists an outdoor coil temperature sensor.^[1][2]

• A normal defrost pauses the heat. Mitsubishi’s FX owner’s manual says the pause is completed in 10 minutes at most.^[3]
• A failed sensor shows as a code. MRCOOL’s manual lists EC 52 for its outdoor coil sensor and EC 50 for an open or short circuit in its outdoor sensors.^[2]
• Testing one is a technician’s job. Mitsubishi’s service manual has the technician measure the sensor’s resistance with a multimeter.^[1]

In our judgement, if a code shows or ice stays on the outdoor unit after a cycle, stop and call a licensed HVAC technician.

What does a heat pump defrost sensor do?

It gives the control board a temperature to read while the unit heats. In Mitsubishi’s MUZ-FX service manual, the defrost thermistor (RT61) is one of the outdoor unit’s thermistors, and the sensor table lists its purpose as “Heating: Defrosting.”^[1] The same manual gives the defrost method as reverse cycle.^[1]

• Mitsubishi calls it a defrost thermistor. It sits beside the fin, ambient, outdoor heat exchanger and discharge thermistors.^[1]
• MRCOOL calls it an outdoor coil temperature sensor. Its manual labels it T3, never calls any sensor a defrost sensor, and does not say which one starts a defrost cycle.^[2]
• Other brands name and test it their own way. We read only these two makers’ manuals for this post, so check the manual for your model.

What does a normal defrost cycle look like?

A normal defrost pauses the heat for a few minutes. Mitsubishi’s FX owner’s manual says frost forms when the outside air is too cold and too humid. Heating can then stop for about 10 minutes, and the defrost is completed in 10 minutes at most.^[3]

• Vapor and dripping outside. Mitsubishi says defrost vapor looks like white smoke and melting ice drips down.^[3]
• White mist and a hiss. MRCOOL says mist after a heat restart comes from defrost moisture, and a low hiss while defrosting is refrigerant gas changing direction.^[2]
• A slow return to warm air. If you start heating while the outdoor unit is defrosting, Mitsubishi says warm air takes a few minutes, 10 at most.^[3]
• A defrost mark on the display. MRCOOL’s indoor display shows DF while the unit defrosts.^[2]

Neither manual says what the outdoor fan does during defrost, so we do not describe it. Our Mitsubishi not heating post walks the owner checks in order.

How can you tell a defrost sensor has failed?

Usually by a code, not by eye. MRCOOL’s manual lists EC 52 for the outdoor coil temperature sensor (T3) and EC 50 for an open or short circuit in the outdoor temperature sensors.^[2] Mitsubishi’s service manual lists a defrost thermistor that shorts or opens among the outdoor thermistor faults, and its owner’s manual lists a defrost thermistor among the outdoor error codes.^[1][3]

• A code means stop. Mitsubishi’s owner’s manual says to stop using the unit, turn off the breaker and call your dealer.^[3]
• No code does not clear the sensor. Constant defrost can have other causes. One reviewer in our buyer-research scan traced excessive, noisy defrost to a control board, fixed by a board replacement.
• A long pause is worth a call. A Mitsubishi FX pause past its 10-minute limit is a reason to tell your installer.^[3]

What can you check yourself, and where does DIY end?

Where DIY ends: the sensor, refrigerant and 208/230V

Checking a defrost sensor means opening the outdoor unit, reaching the board connectors and measuring resistance with a multimeter.^[1] That is a licensed HVAC technician’s job. Mitsubishi’s service manual also has technicians check the defrost heater with 208/230 V AC at the board.^[1] The 208/230V circuit is a licensed electrician’s job, and your local building department decides what needs a permit.

Owner checks stop short of the sensor:

• Do not speed up defrost. Both makers say not to accelerate it by any means they do not recommend.^[3][2]
• Read the display. Write down any code, then stop.
• Check the basics first. Mode, set point and filters come before any sensor worry, as in our mini split maintenance guide.
• Time the pause. Note how long the heat stays off, and compare it with your manual.

If the room still will not hold temperature on cold nights, the unit may be undersized, and a load calculation settles that after the sizing calculator screens it.

How much heat a unit still makes in the cold is a separate question. Our cold climate mini split heat pump guide compares NEEP capacity at 5°F for the major brands.