Bosch Error Code P21: Low pressure protection (Heat Pump)
Bosch error code P21 on a heat pump means: Low pressure protection. It is also called Bosch diagnostic code P21.
Homeowner? What Bosch code P21 means for you
Call a technician Your heat pump is reporting: Low pressure protection. The heat pump has locked out or cannot run properly. One reset is fine; if the code comes back, it needs a licensed technician to find the cause.
Safe things you can check yourself
- Check the thermostat: right mode (Heat or Cool), set a few degrees past room temperature, and fresh batteries if it uses them.
- Check both breakers and the outdoor disconnect box beside the outdoor unit; all should be ON.
- Look at the air filter. If it is grey and clogged, replace it; a blocked filter causes many shutdowns.
- In winter, keep snow and ice off and away from the outdoor unit. A short defrost cycle with steam is normal.
- Try one reset: turn the power off for 30 seconds, then back on. If the code comes back, stop resetting and call a technician.
Call (647) 657-2977Book a service call
Licensed technicians serving Vaughan, Toronto and the GTA.
What does Bosch error code P21 mean?
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
Common causes of Bosch code P21
- Closed service valves
- Undercharge
- Blockage or poor heat exchange
A technician checks these and 1 other possible causes on site before replacing any part.
Bosch models that use code P21
BOVA15-24BOVA15-36BOVA15-60BOVA36-20BOVA60-20
Other Bosch Heat Pump codes
All Bosch Heat Pump error codes →
Bosch code P21: homeowner questions
What does Bosch code P21 mean?
Low pressure protection. Low-side pressure fell below the limit. Five trips in 100 minutes cause lockout F2.
Is it safe to keep using my heat pump with code P21?
It depends on the cause. The heat pump has locked out or cannot run properly. One reset is fine; if the code comes back, it needs a licensed technician to find the cause.
Can I reset Bosch code P21 myself?
You can try one reset (power off for 30 seconds, then on). If Bosch code P21 comes back, the cause is still there and a licensed technician should check it.
Who can fix Bosch code P21 near me?
Aurora Star Tech Inc. repairs Bosch heat pumps in Vaughan, Toronto and across the GTA. Call (647) 657-2977 or book a service call online.
Homeowner help: AC Running But Not Cooling: What to Check · Heat Pump Not Heating or Blowing Cold Air
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. Written in our own words as a summary; always check the original manual.
General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.