ACiQ mini split error code F1
2 meanings on ACiQ, depending on which series you have. Check the model number on the indoor unit's label.
If yours is a standard single-zone and multi-zone (ACiQ-09Z-HP…, Midea-built) unit
Outdoor ambient temperature sensor (T4) open or short circuit.
- TypeFault
- SystemTemperature sensor
- Can I fix it?Homeowner checks, then maybe a tech
What F1 means on a ACiQ
The control board is reading the outdoor ambient temperature sensor (T4) as open or shorted, which means the thermistor (a small resistor whose resistance changes with temperature) is returning a value outside its normal range. The unit usually stops, or runs with the wrong output, because the control board no longer trusts that temperature reading.
On this platform, F1 is a fault: something has failed or is disconnected and needs diagnosis.
Common causes
- The outdoor ambient temperature (T4) thermistor has failed (they do drift and fail with age and heat cycling).
- Its plug has worked loose from the control board, or a pin has corroded.
- The sensor lead is chafed, pinched or chewed, giving an intermittent open or short.
- The control board's sensor input has failed (rare compared with the sensor itself).
How to fix it yourself
Work through the checks below first; they resolve most cases. If they do not, the repair itself may need a technician.
- Reset the system: switch the breaker (or the outdoor disconnect) off, wait a full five minutes so the boards fully discharge, then switch it back on and restart from the remote.
- If the code comes straight back after a reset, the fault is real and not a glitch.
- With power off, locate the outdoor ambient temperature sensor (T4) clipped to the outdoor unit's coil guard or bracket, sensing outside air. Follow its lead back to the board and reseat the plug firmly.
- Inspect the lead for damage along its whole length, especially where it passes through the chassis.
- If you have a multimeter, unplug the sensor and measure its resistance. A reading near zero ohms (short) or infinite (open) confirms a bad sensor. Most mini split thermistors read roughly 5 to 20 kΩ at room temperature, so a value in that range with a steady drift as you warm it in your hand means the sensor is fine and the board is the suspect.
- Replacement thermistors are inexpensive and plug in. Match the part number from the parts list for your model.
Switch off power at the breaker before opening either unit. Capacitors in the outdoor unit hold a charge for several minutes after power is cut.
When to call a technician
Sensor swaps are a doable DIY job on most units. Call a technician if the sensor tests good and the code persists, because that points at the control board.
If yours is a K-series (ACiQ-K09W-HP…, TCL-built) unit
Power test module fault.
- TypeFault
- SystemControl board
- Can I fix it?Technician job
What F1 means on a ACiQ
The outdoor board's power-monitoring module failed its self-test.
On this platform, F1 is a fault: something has failed or is disconnected and needs diagnosis.
Common causes
- A failed module or board.
- A surge event.
How to fix it yourself
Do the reset and the visual checks, but the underlying repair needs a technician with a meter and the service manual.
- Reset the system: switch the breaker (or the outdoor disconnect) off, wait a full five minutes so the boards fully discharge, then switch it back on and restart from the remote.
- If you own a multimeter and are comfortable using it, measure the supply voltage at the outdoor unit's L1 and L2 terminals with the unit running. A 230 V system should read roughly 208 to 253 VAC.
Switch off power at the breaker before opening either unit. Capacitors in the outdoor unit hold a charge for several minutes after power is cut.
When to call a technician
Board-level fault.
Which ACiQ units show F1
ACiQ sells Standard single-zone (ACiQ-09Z-HP…B / W-HP) and multi-zone and K-series (ACiQ-K09W-HP…, SZDM / WHP / KZHP) units, and this code carries more than one meaning across them: on the standard single-zone and multi-zone (ACiQ-09Z-HP…, Midea-built), F1 means outdoor ambient temperature sensor (T4) open or short circuit; on the K-series (ACiQ-K09W-HP…, TCL-built), F1 means power test module fault. The model number on the indoor unit's label, and which display you are reading, tell you which applies.
Where ACiQ shows this code
standard single-zone and multi-zone (ACiQ-09Z-HP…, Midea-built): The code appears on the indoor unit's LED display in place of the set temperature. Some models also blink the indicator lamps in a pattern that matches the code. E codes are faults in the indoor unit or its communication link, F codes are faults on the outdoor side, and P codes are protections: the outdoor unit has stopped itself to prevent damage. EC is the refrigerant leak check.
K-series (ACiQ-K09W-HP…, TCL-built): The indoor display shows the code in place of the set temperature. E codes are faults: something has failed and needs diagnosis. P, H, d and F codes are protections: the system stopped or throttled itself to prevent damage, and many clear on their own once conditions return to normal.
Other ACiQ codes
Same part of the system as F1:
F1 on other brands
The same characters do not always mean the same thing. Check the brand on your unit.
- Pioneer F1 Outdoor ambient temperature sensor (T4) open or short circuit / Zone A indoor coil outlet sensor (T2B) fault, shown on the outdoor display
- Blueridge F1 Indoor ambient (room) temperature sensor fault (open or short circuit) / Outdoor ambient temperature sensor (T4) open or short circuit
- Gree F1 Indoor ambient (room) temperature sensor fault (open or short circuit)
- Zone Air F1 Power test module fault
- Carrier F1 Outdoor ambient temperature sensor (T4) open or short circuit
- Heil F1 Outdoor ambient temperature sensor (T4) open or short circuit / Indoor ambient (room) temperature sensor fault (open or short circuit)
Questions people ask
Why does F1 have more than one meaning on a ACiQ?
ACiQ sells units built on more than one control platform, and each platform has its own code table. The model number on the indoor unit's label tells you which series you have: standard single-zone and multi-zone (ACiQ-09Z-HP…, Midea-built) units read F1 as outdoor ambient temperature sensor (T4) open or short circuit; K-series (ACiQ-K09W-HP…, TCL-built) units read F1 as power test module fault.
Can I reset a ACiQ F1 code myself?
Yes. Turn the system off at the breaker for five minutes, then restart it. If F1 comes back, the fault is real and needs the checks above.
Is it safe to keep running a ACiQ showing F1?
The unit will usually refuse to run, or run with reduced output, until the fault is cleared. Repeated resets do not fix anything and can mask a problem that is getting worse.
Sources
- ACiQ Standard Single-Zone Mini Split Service Manual (SZ-Z-HP / W-HP), Quick Maintenance by Error Code
- ACiQ Mini-Split Air Handler and Condenser Service Manual (MS-SZDM / WHP / KZHP), error code table
Updated September 2026. Codes and definitions vary by model year. If your manual disagrees with this page, your manual wins. Send a correction.