F4
Blue Star Split AC
Severity: ModerateWhat it means
F4 on a Blue Star AC means the outdoor coil (condenser pipe) temperature thermistor has a fault.
This sensor monitors the outdoor heat exchanger temperature to protect the compressor and control the refrigerant cycle.
A failed sensor triggers F4 and the system stops.
Affected Models
- Blue Star Inverter Split AC (all capacities)
- Blue Star 3 Star Inverter
- Blue Star 5 Star Inverter
- Blue Star FS Series
- Blue Star IC Series Inverter
Common Causes
- Outdoor coil thermistor failed — resistance outside the normal operating range
- Thermistor clip dislodged from the condenser coil pipe on the outdoor unit
- Thermistor wiring corroded or broken from weather exposure
- Connector at the outdoor PCB loose or humidity-damaged
- Outdoor PCB fault misreading the coil sensor signal
How to Fix It
-
Switch off at the remote and circuit breaker. Wait 5 minutes and restart.
If F4 clears and the AC runs normally, monitor for recurrence.
A persistent F4 confirms the thermistor has failed. -
Ensure the outdoor unit has full ventilation clearance.
Clear at least 30–50 cm on all sides.
An obstructed outdoor unit increases coil temperature, which can push a marginal thermistor into fault range. -
Call Blue Star service (1800-209-1177) if F4 persists.
The technician will test outdoor coil thermistor resistance and replace if faulty.
When to Call a Professional
F4 requires a Blue Star authorised technician — do not open the outdoor unit.
Blue Star service helpline: 1800-209-1177.
Frequently Asked Questions
Is F4 on Blue Star AC the same as F3?
They are related but different.
F3 is typically the outdoor ambient or coil sensor.
F4 is a different sensor in the outdoor unit — on most Blue Star models, this is the discharge pipe or a second outdoor coil thermistor.
Both require a technician to diagnose and replace the correct sensor.