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Bosch · Heat pump

P21 Low pressure protection

High severity Bosch IDS LightBosch IDS Premium Connected

What the control is telling you

Low-side pressure fell below the limit. Five trips in 100 minutes cause lockout F2.

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

  • Closed service valves
  • Undercharge
  • Blockage or poor heat exchange
  • Failed transducer

Test and repair steps

  1. Open the service valves.
  2. Check the refrigerant charge.
  3. Clear blockages and fix airflow or fan problems.
  4. Compare suction pressure with the point check value; replace the transducer if off.

Models that show this code

5 models in Bosch IDS Light, Bosch IDS Premium Connected

BOVA15-24BOVA15-36BOVA15-60BOVA36-20BOVA60-20

Other codes on the same units

  • P1 High pressure protection
  • P4 High compressor discharge temperature
  • P5 Condenser coil temperature protection (cooling)
  • P11 High pressure protection (transducer)
  • P31 Input over-current protection
  • P32 Compressor over-current protection
  • AtL Ambient temperature limited
  • EA Control and drive program mismatch

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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.