Thermistors and temperature sensors
The cheap sensors that tell the board how hot everything is — inlet water, outlet water, heat exchanger, incoming air, and on Rinnai's SENSEI RX, a dedicated lime sensor. Individually inexpensive; collectively behind a large share of error codes.
What they do
A thermistor is a resistor whose resistance changes predictably with temperature. The board reads that resistance and converts it to a temperature. A tankless heater carries several: inlet water, outgoing water, heat exchanger, incoming air, and a freeze protection sensor.
One of them is filed with these and does not belong here. The sensor in the flue does a different job, sits in the exhaust path of a gas appliance, and is treated as professional work by every manufacturer in our archive — it has its own page, the exhaust gas temperature sensor.
They are checked the same way everywhere. Rinnai publishes the expected values for its standard sensors — roughly 11.4-14K ohms at 59°F falling to 2.2-2.7K ohms at 140°F — and a technician compares the sensor against that table before condemning it.
The SENSEI RX exception worth knowing
Rinnai’s SENSEI RX generation adds a lime detecting thermistor clamped to the hot water pipe, and it is not the same kind of sensor: its range runs around 70.6-85.0K ohms at 59°F, five times the standard sensors. It is the sensor behind the LC scale lockout, and it is why code 34 means something completely different on an RX than on Rinnai’s older units, where that same number refers to a combustion air temperature sensor.
Why they fail
Scale, mostly. Any sensor sitting in the water path collects the same mineral deposit that coats the heat exchanger, and a coated sensor reads slow and wrong. Rinnai’s step before replacing a water-side thermistor is to remove it and clean the build-up off. The rest is broken wiring, a crushed connector, or moisture in the harness.
Why this is not a DIY part
The part is trivially cheap; getting to it is not. Water-side sensors need the supply isolated and the unit drained, with an O-ring that has to be intact and lubricated on reassembly, and a leak check afterwards.
What a replacement usually costs
$20-80 for the sensor. Realistically $150-450 done, because you are paying for a visit, a drain-down, and a leak test.