Bosch Error Code d3: High limit / temperature limiter circuit open (Boiler)
Bosch error code d3 on a boiler means: High limit / temperature limiter circuit open. It is also called Bosch diagnostic code d3.
Homeowner? What Bosch code d3 means for you
Call a technician Your boiler is reporting: High limit / temperature limiter circuit open. The boiler 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.
- Make sure the unit has power: its on/off switch (often looks like a light switch near the unit) is ON and the breaker in the electrical panel is not tripped.
- Look at the pressure gauge on the boiler. If it reads much lower than usual or near zero, or water is leaking, call a technician.
- Outside, make sure the intake and exhaust pipes or vent cap are not blocked by snow, ice, leaves or a nest.
- Check that the boiler gas valve is open (handle in line with the pipe).
- 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 d3 mean?
The temperature limiter circuit is open or an external limit has tripped, locking the boiler out.
Applies to: Bosch Greenstar ZBR 21/28/35/42-3A boilers and ZWB 28/35/42-3A combi boilers
Common causes of Bosch code d3
- Sensor or lead open or shorted
- Temperature guard TB1 tripped or jumper missing across 8-9 or PR-P0
- External limit, LWCO or pressure device tripped
A technician checks these and 1 other possible causes on site before replacing any part.
Bosch models that use code d3
ZBR21-3AZBR28-3AZBR35-3AZBR42-3A
Other Bosch Boiler codes
All Bosch Boiler error codes →
Bosch code d3: homeowner questions
What does Bosch code d3 mean?
High limit / temperature limiter circuit open. The temperature limiter circuit is open or an external limit has tripped, locking the boiler out.
Is it safe to keep using my boiler with code d3?
It depends on the cause. The boiler 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 d3 myself?
You can try one reset (power off for 30 seconds, then on). If Bosch code d3 comes back, the cause is still there and a licensed technician should check it.
Who can fix Bosch code d3 near me?
Aurora Star Tech Inc. repairs Bosch boilers in Vaughan, Toronto and across the GTA. Call (647) 657-2977 or book a service call online.
Homeowner help: Boiler Not Heating: What to Check
Troubleshooting guides
- No heat or some zones cold All brands · Boiler
Source: Bosch Greenstar ZBR/ZWB 21-42-3A service manual 6721853955 (2025-11), Table 38 Faults shown on the display, p.74-75; Greenstar combi 100 p/151 p service manual 6721853953 (2025-11), Table 33. 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.