Rinnai SENSEI Code 51
Inlet water temperature thermistor fault. The sensor reading the cold water entering the heater is out of range. Like code 41, this one does not stop the unit: Rinnai notes that if the inlet thermistor is open or disconnected the heater keeps running and flashes code 51, alternating with the set temperature, until the cause is cleared. With no code 34 on this generation, the inlet sensor at 51 and the pair at 32 and 33 are the whole water-side sensor set. Every temperature decision the heater makes comes from those three, which is why an inlet fault that lets the unit keep running is still worth clearing.
Part replacementThe verdict
Replace a part
An inexpensive sensor the heater can limp without — but it is the reference the unit uses to decide how hard to fire, so expect temperature to wander at the tap until it is fixed. Rinnai's step before replacing it is to clean scale off it.
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 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 whether water temperature has become inconsistent at the tap — useful detail for the technician.
- Note when the unit was last flushed; scale on the sensor is checked before it is condemned.
- Book a service call.
Frequently asked questions
The display alternates between 120 and 51. Normal for this code?
Yes — Rinnai documents exactly that: the display alternates between the set point and code 51 while the heater keeps operating.
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
2 other Rinnai 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: