F4
Blue Star Split AC
Severity: ModerateWhat Does This Error Mean?
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
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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.
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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.
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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.