ACiQ mini split error code P4
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
Inverter compressor drive error.
- TypeProtection
- SystemCompressor / inverter
- Can I fix it?Technician job
What P4 means on a ACiQ
The inverter tried to drive the compressor and lost control of it: the compressor would not start, lost sync, or drew current the drive could not regulate. The outdoor unit stops.
On this platform, P4 is a protection: the system stopped or throttled itself to prevent damage; many clear on their own once conditions return to normal.
Common causes
- A locked or failing compressor.
- Loose or corroded compressor terminals (a loose spade under the terminal cover is common on older units).
- A failed inverter drive board or IPM.
- Low supply voltage during compressor start.
- Liquid refrigerant flooding the compressor after it has sat in cold weather.
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.
- Listen at the outdoor unit on restart. A hum with no compressor start, or a short buzz then silence, points to the compressor or its drive.
- Do not repeatedly reset and retry. Each failed start stresses the drive.
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
Compressor and inverter drive diagnosis is a technician job. Ask for compressor winding resistance and drive output checks before agreeing to a compressor replacement.
If yours is a K-series (ACiQ-K09W-HP…, TCL-built) unit
Excessive outdoor discharge temperature.
- TypeProtection
- SystemCompressor / inverter
- Can I fix it?Homeowner checks, then maybe a tech
What P4 means on a ACiQ
The refrigerant leaving the compressor got too hot. The compressor stops to protect itself.
On this platform, P4 is a protection: the system stopped or throttled itself to prevent damage; many clear on their own once conditions return to normal.
Common causes
- Low refrigerant charge.
- A blocked outdoor coil or failed fan.
- A partly closed service valve.
- A failing compressor.
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.
- Look at the outdoor unit: clear leaves, cottonwood fluff, snow or ice from the coil and make sure the fan can spin freely and nothing is stacked against the cabinet.
- Confirm the service valves are fully open.
- Cool down with power off for 30 minutes and retry.
- Anything inside the refrigerant circuit (sensors are fine, but valves, compressors and leaks are not) needs EPA-certified refrigerant handling. That is a technician job.
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
Recurring P4 with a clean coil needs a charge check.
Which ACiQ units show P4
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), P4 means inverter compressor drive error; on the K-series (ACiQ-K09W-HP…, TCL-built), P4 means excessive outdoor discharge temperature. 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 P4:
P4 on other brands
The same characters do not always mean the same thing. Check the brand on your unit.
- Pioneer P4 Inverter compressor drive error / Compressor discharge temperature protection (outdoor display)
- Blueridge P4 Inverter compressor drive error
- Zone Air P4 Excessive outdoor discharge temperature
- Daikin P4 Inverter heat sink temperature sensor fault
- Mitsubishi Electric P4 Drain sensor or float switch fault
- Trane P4 Excessive outdoor discharge temperature / Drain sensor or float switch fault
Questions people ask
Why does P4 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 P4 as inverter compressor drive error; K-series (ACiQ-K09W-HP…, TCL-built) units read P4 as excessive outdoor discharge temperature.
Can I reset a ACiQ P4 code myself?
Yes. Turn the system off at the breaker for five minutes, then restart it. If P4 comes back, the fault is real and needs the checks above.
Is it safe to keep running a ACiQ showing P4?
Not repeatedly. A protection code means the unit stopped itself to prevent damage, and resetting it over and over can turn a cheap repair into a compressor failure. Reset once, check the basics, and stop if it returns.
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.