HVACopediaBosch · Heat pump
b4 Indoor refrigerant sensor communication fault
What the control is telling you
The control cannot communicate with the A2L sensor.
Applies to: Bosch IDS Light BOVA15-24/36/60 and IDS Premium Connected BOVA36-20/60-20 R-454B heat pump outdoor units
Likely causes
- Sensor unplugged or not seated
- Failed sensor
Test and repair steps
- Reseat the sensor and its wiring.
- Replace the sensor and repower if it persists.
Models that show this code
5 models in Bosch IDS Light, Bosch IDS Premium Connected
Other codes on the same units
- b2 Indoor T2 sensor fault
- b3 Indoor refrigerant sensor hardware fault
- b5 Indoor to outdoor communication fault
- b7 R-454B refrigerant leak protection
- b8 Refrigerant sensor past service life
- C3 T3 coil sensor not reading correctly in cooling
- E7 T5 discharge sensor not reading correctly
- E41 T3 temperature sensor fault
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Source: Bosch IDS Light installation BTC 762003308 D (02.2025), Fault Code Table 23 and 17.12 Error Code Troubleshooting, p.42-59; Bosch IDS Premium Connected installation BTC 762003301 D (02.2025), Fault Code Table 22 and 18.12 Error Code Troubleshooting. Summarized in our own words; check the original manual for your model.
Homeowner looking at this code? See what it means for you and when to call a technician.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.