Rinnai SENSEI RX Code 51
Inlet water temperature thermistor fault. The sensor reading the temperature of 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.
Part replacementThe verdict
Replace a part
An inexpensive sensor that 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, so on hard water a flush may be part of the visit.
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 RX — 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 the 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 Rinnai's step before condemning it.
- Book a service call.
Frequently asked questions
The display alternates between 120 and 51. Is that normal for this code?
Yes — Rinnai documents exactly that behaviour: the display alternates between the set point and code 51 while the heater keeps operating.