Rinnai Error Code 303: Secondary system thermistor fault (Boiler)
Rinnai error code 303 on a boiler means: Secondary system thermistor fault. It is also called Rinnai diagnostic code 303.
Homeowner? What Rinnai code 303 means for you
Check, then call Your boiler is reporting: Secondary system thermistor 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 Rinnai error code 303 mean?
Fault in the secondary system temperature thermistor used for cascade heating control between boilers.
Applies to: I-Series Plus boilers — 175K BTU and higher (Combi and Solo)
Common causes of Rinnai code 303
- Parameter 70 not set to 'b' (in use)
- Loose, broken or damaged thermistor wiring/connectors
- Thermistor unplugged (reads -40)
A technician checks these and 2 other possible causes on site before replacing any part.
Rinnai models that use code 303
Other Rinnai Boiler codes
All Rinnai Boiler error codes →
Rinnai code 303: homeowner questions
What does Rinnai code 303 mean?
Secondary system thermistor fault. Fault in the secondary system temperature thermistor used for cascade heating control between boilers.
Is it safe to keep using my boiler with code 303?
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 Rinnai code 303 myself?
You can try one reset (power off for 30 seconds, then on). If Rinnai code 303 comes back, the cause is still there and a licensed technician should check it.
Who can fix Rinnai code 303 near me?
Aurora Star Tech Inc. repairs Rinnai 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: Rinnai Troubleshooting I-Series Plus Boiler Diagnostic Codes (800000375), p.15–17. 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.