Bosch Error Code E3: Additional supply temperature sensor fault (Boiler)
Bosch error code E3 on a boiler means: Additional supply temperature sensor fault. It is also called Bosch diagnostic code E3.
Homeowner? What Bosch code E3 means for you
Check, then call Your boiler is reporting: Additional supply temperature sensor fault. This is often caused by something simple. Try the safe checks below; if the code returns, book a service call before it turns into a no-heat or no-hot-water emergency.
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 E3 mean?
The extra supply temperature sensor reads as faulty.
Applies to: Bosch Greenstar ZBR 21/28/35/42-3A boilers and ZWB 28/35/42-3A combi boilers
Common causes of Bosch code E3
- Open or shorted sensor or lead
Bosch models that use code E3
ZBR21-3AZBR28-3AZBR35-3AZBR42-3A
Other Bosch Boiler codes
All Bosch Boiler error codes →
Bosch code E3: homeowner questions
What does Bosch code E3 mean?
Additional supply temperature sensor fault. The extra supply temperature sensor reads as faulty.
Is it safe to keep using my boiler with code E3?
It depends on the cause. This is often caused by something simple. Try the safe checks below; if the code returns, book a service call before it turns into a no-heat or no-hot-water emergency.
Can I reset Bosch code E3 myself?
You can try one reset (power off for 30 seconds, then on). If Bosch code E3 comes back, the cause is still there and a licensed technician should check it.
Who can fix Bosch code E3 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.