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Rinnai · Tankless water heater

51 Inlet water temperature thermistor fault

Medium severity Non-SENSEIRH180 hybridSENSEI RX/CXSENSEI

What the control is telling you

The inlet (incoming) water temperature thermistor is open, disconnected or reading abnormally. The heater keeps running and flashes 51 alternately with the set point.

Applies to: SENSEI Series; SENSEI RX/CX Series; Non-SENSEI Series

Likely causes

  • Loose, broken or damaged sensor harness
  • Scale build-up on the sensor
  • Sensor open or disconnected
  • Sensor resistance out of spec / failed sensor

Test and repair steps

  1. Inspect the sensor harness from sensor to PC board for loose, broken or damaged connections.
  2. Caution: Shut off the water supply and drain the system before continuing.
  3. With the lines empty, remove the sensor and clean off any scale or deposits.
  4. With a meter set for 20K ohms, measure across the sensor plug; resistance should drop when warmed and rise when cooled.
  5. Typical values 11.4–14K ohms at 59°F, 6.4–7.8K at 86°F, 3.6–4.5K at 113°F, 2.2–2.7K at 140°F, 0.6–0.8K at 221°F.
  6. If readings are good, reinstall the sensor, making sure the small O-ring in the sensor well is intact and lightly lubricated.
  7. Restore water and check for leaks around the sensor.
  8. Fire the unit again; if code 51 returns, replace the sensor. The code clears on its own once the fault is corrected.

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Source: Rinnai Troubleshooting TWH Diagnostic Codes (100000652), p.45. Summarized in our own words; check the original manual for your model.

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