Exhaust gas temperature sensor
The thermistor that sits in the flue rather than in the water. It costs what the other sensors cost and it is not the same job: it is the only part watching for an exhaust temperature that says combustion has gone wrong, and every brand on this site treats a fault in it as professional work.
What it does
Every tankless heater on this site carries several thermistors — inlet water, outlet water, heat exchanger — and one that is filed with them and does not belong with them. The exhaust sensor sits in the flue path, downstream of the burner, and its reading is not used to control your water temperature at all. It exists to answer one question: is the exhaust hotter than this appliance should ever produce?
That single reading is what stands behind a whole family of codes — Rinnai’s 54, Noritz’s 94, Takagi’s 941, Navien’s E047. When the flue runs hot, the heater stops.
Why an overheating flue matters
A condensing water heater is built to pull so much heat out of its exhaust that the water vapour in it condenses back to liquid — which is why it has a condensate drain at all. Noritz states the threshold outright on the NRCP range: the unit shuts down when the flue temperature exceeds 149°F (65°C). That is a low number by the standards of a gas appliance, and deliberately so.
Heat leaving through the vent is heat that did not go into your water, so a hot flue usually means the heat exchanger has stopped transferring it — scale, most often. The other cause is combustion that has drifted, and that one is worse: a vent system is sized and specified for a given exhaust temperature, and running hotter than that puts heat into materials chosen on the assumption it would not happen.
Why it is never a DIY part
The sensor is cheap and the labour is not the reason either. It sits in the exhaust stream of a gas appliance, so reaching it means opening the flue path, and putting it back means the joint has to be sound afterwards — a badly reseated exhaust connection leaks combustion products into the building rather than outside it. Every manufacturer in our source archive routes this code to a technician, and several put it inside a “licensed professional only” block.
There is a second reason. While this sensor is open, shorted or out of range, the appliance has lost the input it uses to notice its own exhaust overheating. The part that is broken is the one that was watching.
The number is not the same across brands, or even within one
This is the sensor behind Rinnai’s code 38 — except on the RH180 hybrid, where Rinnai heads the same number “CO or FV Sensor” and means a carbon monoxide detector. Same manufacturer, same number, a completely different component and a completely different level of urgency. It is the clearest example on this site of why a code has to be read against the exact model.
On Noritz, the sensor fault is 35 on the NRC and EZ ranges and Er35 on the NRCP, and the overheating it detects is 94 or Er94. Navien calls the fault E047 and the part an exhaust thermostat. Takagi splits them into 341 for the sensor and 941 for the temperature.
What a replacement usually costs
$25-90 for the sensor. Realistically $150-500 fitted, because the price is the visit and the flue work rather than the part — and because a technician who finds this code will usually want to know why the exhaust was hot before agreeing the sensor was wrong.