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
The same code number can mean something different on another series — make sure you're reading the page for your exact unit: