Panasonic WH-UD28AE Service Manual page 59

16.5.17. Four Way Valve Abnormality (F11)
Malfunction Decision Conditions:
During operation of heating, temperature below 0°C is detected by indoor pipe temperature sensor.
Malfunction Caused:
1. Connector in poor contact.
2. Faulty sensor.
3. Faulty outdoor unit PCB.
4. Four way valve defective.
Abnormality Judgment:
Continue 4 times in 30 minutes.
59