MRCOOL air conditioner F1 error code: outdoor ambient temperature sensor fault

What the F1 code means

F1 on a MRCOOL single-zone mini-split means the outdoor ambient temperature sensor (T4) has an open or short circuit. Gen 4 and Gen 5 units show it as EC 53. The unit uses this sensor to adjust to the weather, so it stops when the reading is lost.

The T4 sensor sits on the back of the outdoor unit and reads outdoor air temperature. The board uses it to set compressor and fan speed and to decide when to defrost in heating.

MRCOOL lists other F codes on the same family of single-zone units: F2 for the condenser coil sensor and F3 for the discharge sensor. They are fixed the same way.

Common causes

Listed from most to least common.

1. Outdoor ambient sensor failed
2. Sensor plug loose on the outdoor control board
3. Sensor wire broken or corroded
4. Outdoor control board fault

How to fix the F1 code

Unplug the air conditioner or switch it off at the breaker before removing any panel or filter.

1. 1

   Step 1: Reset the power

   Turn off the breaker for 15 minutes, then turn it back on and watch for the code.

2. 2

   Step 2: Clear around the outdoor unit

   Remove leaves, dirt or snow from around the unit. MRCOOL notes that debris and long direct sun can affect the sensor.

3. 3

   Step 3: Book a repair if it returns

   The sensor plugs into the outdoor board. Checking it means opening the outdoor unit, so have a technician do it.

How to reset the MRCOOL air conditioner

1. Turn the mini-split off with the remote.
2. Switch off the breaker for the system and the outdoor disconnect switch. MRCOOL suggests leaving the power off for 15 minutes so the boards fully reset.
3. Restore power and turn the unit back on. The compressor normally waits about 3 minutes before starting, so give it time before judging.
4. If the same code comes back straight away, the fault is still there. Work through the fix steps above, or call a technician.

When to call a technician

If F1 returns after a reset, a technician should check the sensor plug and test the sensor's resistance. If the sensor is fine, the outdoor board is next.

A typical repair for this code costs $150–$450 in the US, including parts and labor. Prices vary by area and model.

Models this applies to

• DIY Series ductless mini-splits (Gen 1 to 5), single-zone and multi-zone
• Advantage Series mini-splits
• Olympus Series mini-splits
• EZ-PRO mini-splits

Codes can differ slightly between models. Your owner's manual is the final word and is available from MRCOOL support.

Frequently asked questions