Rinnai · Tankless Water Heater Moderate

51 — Rinnai Tankless Water Heater Code 51 — Inlet Water Temperature Thermistor Fault

The inlet water temperature thermistor is open, disconnected, or reading out of range. The unit keeps operating and alternates the display between the set temperature and code 51.

Keep using it?
Yes, with caution
DIY difficulty
Moderate
Verified against
media.rinnai.us ↗

How to fix it

  1. Power off the unit.
  2. Locate the inlet water temperature thermistor and reseat its connector.
  3. Measure resistance and compare to the manufacturer's spec chart for the measured inlet water temperature.
  4. Replace the thermistor if it's open, shorted, or out of range.
  5. Inspect harness routing for damage.

Why it happens

Typical signs: Display alternates between the set temperature and 51 · Unit continues to operate and deliver hot water · Inlet temperature reading may be inaccurate

  • Failed inlet thermistor Sensor element open or reading out of range.
  • Disconnected or loose connector Sensor plug backed off the PCB header.
  • Wiring damage Chafed or pinched harness between the sensor and the board.

Parts you may need

If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.

  • Rinnai inlet water thermistor Common replacement
    Amazon →

More Rinnai tankless water heater codes

02 Code 02 — No Burner Operation During Freeze Protection 03 Code 03 — Power Interruption During Bath Fill 10 10 — Air Supply or Exhaust Blockage 11 11 — No Ignition 12 12 — Flame Failure 14 14 — Thermal Fuse / Overheat Cutoff

All Rinnai codes → All tankless water heater codes →

Frequently asked questions

What does the 51 code mean on a Rinnai tankless water heater?

The inlet water temperature thermistor is open, disconnected, or reading out of range. The unit keeps operating and alternates the display between the set temperature and code 51.

What causes the 51 code on a Rinnai tankless water heater?

Failed inlet thermistor: Sensor element open or reading out of range. Disconnected or loose connector: Sensor plug backed off the PCB header. Wiring damage: Chafed or pinched harness between the sensor and the board.

Is it safe to keep using a Rinnai tankless water heater with a 51 error?

Yes, with caution. This is rated a moderate-severity fault.

How do I fix 51 on a Rinnai tankless water heater?

Power off the unit. Locate the inlet water temperature thermistor and reseat its connector. Measure resistance and compare to the manufacturer's spec chart for the measured inlet water temperature.

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: media.rinnai.us.