Rinnai Non-SENSEI Code 51
Inlet water temperature thermistor fault. The sensor reading the temperature of water entering the unit is out of range. Rinnai's procedure is the thermistor routine — isolate and drain, remove the sensor, clean scale off it, check it against the resistance curve, and refit it with the O-ring intact and lubricated.
Part replacementThe verdict
Replace a part
Inexpensive part, ordinary service call. If the sensor comes out coated in scale, the sensor is the symptom and the water is the cause.
Typical cost: $150–$450
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. 45), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note when the unit was last flushed — Rinnai's own step is to clean scale off this sensor before replacing it.
- Book a service call; the unit has to be isolated and drained.
Code 51 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:
Don't confuse this with
3 other Rinnai Non-SENSEI codes point at a thermistor 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: