Rinnai Non-SENSEI Code 54
Exhaust gas temperature too high. Rinnai gives the threshold: above 163°F the unit reduces its firing rate, and if the exhaust is still above 163°F after 120 seconds it shuts down and flashes code 54. The causes are vent clearances and restrictions, the wrong gas type, inlet water above 160°F, and — the one that shows up most in hard-water homes — hot spots on the heat exchanger caused by scale inside it. Rinnai's remedy for that is a flush with four gallons of vinegar, and replacement if the exchanger is already damaged.
Call a proThe verdict
Call a licensed plumber
Exhaust temperature is a combustion measurement, so the diagnosis is professional. But the cause is often scale, and that part is preventable and cheap — which is why the low end of this range is a flush and the high end is a heat exchanger.
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. 49), 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.
- Check the vent terminal outside for restrictions and confirm nothing has been built or stacked near it.
- Note when the unit was last flushed, and whether your water is hard. Rinnai names scale in the heat exchanger as a direct cause of this code.
- If the unit is fed from a solar or recirculating source, check the incoming water is not already above 160°F.
- Book a licensed professional.
Code 54 on other Rinnai series
Other Rinnai series report code 54 under the same fault name, but each manual is transcribed separately and they do not read alike:
Don't confuse this with
2 other Rinnai Non-SENSEI codes point at the exhaust path 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: