Carrier mini split error code P4
Inverter compressor drive error.
- TypeProtection
- SystemCompressor / inverter
- Can I fix it?Technician job
- Applies toindoor display, classic two-character codes (38MHRB, 38MGR, 40MAQ)
What P4 means on a Carrier
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.
Which Carrier units show P4
This page applies to Carrier Comfort 38MHRB / 40MHHQ, Comfort 38MGR / 40MGQ, Performance 40MAQ / 38MAQ, Infinity 40MPHA / 38MPRA and Carrier multi-zone units on the indoor display, classic two-character codes (38MHRB, 38MGR, 40MAQ). Carrier's own documentation lists P4 as "Inverter compressor drive error", the same definition used across the Midea platform (classic codes) it is built on. If your unit shows a code with a different shape (for example a four-character code, or a plain number), it is from a different table; use the lookup with your exact code.
Where Carrier shows this code
indoor display, classic two-character codes (38MHRB, 38MGR, 40MAQ): 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.
Other Carrier 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
- ACiQ P4 Inverter compressor drive error / Excessive outdoor discharge temperature
Questions people ask
Can I reset a Carrier 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 Carrier 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.
What will a technician check for P4?
Compressor and inverter drive diagnosis is a technician job. Ask for compressor winding resistance and drive output checks before agreeing to a compressor replacement. Ask for the measurements, not just a parts quote.
Sources
- Carrier 38MHRB Single Zone Ductless Service Manual SG-38MHRB-01 (Diagnosis and Solution section)
- Carrier 40MAQ / 38MAQ High-Wall Ductless Service Manual 40MAQ-01SM (Diagnosis and Solution section)
- Carrier 40MPHA / 38MPRA Infinity High Wall Ductless Service Manual (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.