Rinnai Non-SENSEI Code 58
Secondary heat exchanger switch tripped. One of the two bi-metal switches on the rear of the secondary heat exchanger has opened, where the air box meets the exchanger. Like code 57 it appears whether or not the unit is running and needs a hard reset after the cause is corrected. Rinnai is unusually direct about what that cause is: this code indicates scale build-up inside the secondary heat exchanger, and the unit needs flushing to prevent damage.
Call a proThe verdict
Call a licensed plumber
A safety switch on the exhaust-side heat exchanger, so the call is professional — but the underlying cause Rinnai names is scale, and that is the part you control. If the exchanger is already damaged the repair is at the top of this range.
Typical cost: $150–$1,400
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the Non-SENSEI — not averaged across the brand.
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 51), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
Before anything else: if you smell gas, stop here, leave, and call your gas utility.
- Leave the unit off until it has been looked at.
- Note when it was last flushed and whether the house has hard water — Rinnai names scale build-up as the cause of this code, not as one possibility among many.
- Book a licensed professional, and ask for a flush as part of the visit rather than a separate trip.
Frequently asked questions
Can I just reset code 58 and carry on?
Rinnai's instruction is to correct the cause first and then hard reset. Since the cause it names is scale inside the secondary heat exchanger, resetting without flushing leaves the exchanger cooking in the same conditions that tripped the switch — and a heat exchanger is the most expensive part on the unit.
Don't confuse this with
2 other Rinnai Non-SENSEI codes point at a switch too. They are different components in different places, and ordering a part against the wrong one is the most common way this fault gets expensive: