P4
Carrier Split AC
Severity:What Does This Error Mean?
P4 on a Carrier AC means the discharge pipe temperature sensor has detected overheating from the compressor. This usually points to low refrigerant, a restricted system, or compressor overload. Service is needed — P4 is a sign of system stress and ignoring it risks compressor damage.
Affected Models
- Carrier 24SCA5
- Carrier 38GVQ
- Carrier 53QHET
- Carrier Inverter Split AC
- Carrier Comfort Series
Common Causes
- Low refrigerant from a leak (compressor discharges hotter when undercharged)
- Restricted airflow at the outdoor unit causing high condensing temperature
- Compressor mechanical wear creating excess friction heat
- Failed discharge temperature sensor giving false reading
- Refrigerant restriction from a kinked line or blocked filter drier
How to Fix It
-
Power off and let the unit cool.
Switch off at the breaker for 30 minutes. The discharge sensor needs to cool below the trip threshold before the unit will start. Forcing restarts before cooling can damage the compressor.
-
Check outdoor airflow.
Clean the outdoor coil and clear anything within 30 cm of the unit. Restricted airflow raises condensing pressure and discharge temperature, triggering P4 in hot weather even without other faults.
-
Listen for unusual compressor sounds.
On restart, listen for grinding, knocking, or scraping from the compressor. Healthy compressors sound smooth. Mechanical noise suggests internal wear contributing to high discharge temperature.
-
Note recent service history.
P4 within weeks of a top-up may indicate residual undercharge. P4 in an older unit often means a slow leak. The technician needs this history for efficient diagnosis.
-
Book Carrier service immediately.
P4 should not be ignored. Service typical cost: USD 150–250 for diagnosis. If a leak is found, repair adds USD 200–600 depending on location. Compressor replacement (worst case) USD 800–1,500.
When to Call a Professional
P4 needs a refrigerant pressure check and discharge temperature measurement — work that requires HVAC manifold gauges and refrigerant handling certification. Book Carrier-authorised service.
Frequently Asked Questions
Why does P4 only appear in hot weather?
Hot weather raises both ambient and condensing temperatures, multiplying the effect of any underlying issue (low charge, dirty coil). A system that is marginal in mild weather hits the P4 threshold when ambient hits 35°C+. The fix is the same — but P4 reveals weakness.
Will my Carrier AC restart on its own after P4?
Most Carrier ACs auto-restart after the discharge sensor cools — typically 20–40 minutes. The auto-restart is not a fix, only a delay. The underlying cause must be addressed or P4 will return repeatedly.