Midea M-Thermal Arctic HT R290 Mono fault BA

What it means

T4 sensor detected temperature outside of normal operation range. In heating/DHW mode, the error occurs when T4≥65℃ or T4<-25℃. In cooling mode, the error occurs when T4≥65℃ or T4<-5℃.

Likely causes

1. T4 sensor reading out of range
2. T4 sensor fault
3. Low ambient temperature (T4<3°C) combined with low water-side temperature triggers freeze protection
4. Water side temperature dropping below critical thresholds due to low flow, low ambient temperature, or system stoppage
5. Pump stalling due to impeller obstruction
6. Feedback signal cable CN17 not connected properly

Where to start

1. Check T4 (ambient temperature) reading against 3°C threshold
2. Check Min(Twout,Twin,T1) against 4°C threshold sustained for 5 seconds
3. Check Min(Twout,Twin,T1) against 2°C threshold (immediate trigger)