Rinnai tankless water heater code 38
Your series decides which reading applies. Across the 4 Rinnai series documented here, code 38 is listed as exhaust gas temperature sensor fault (3 series) and carbon monoxide or flammable vapour sensor fault (1 series). Find your series in the table below, where those readings route to a professional.
What code 38 means on each Rinnai series
| Series | What it means | Verdict | Typical cost |
|---|---|---|---|
| Non-SENSEI | Exhaust gas temperature sensor fault | Call a pro | $150–$600 |
| RH180 Hybrid | Carbon monoxide or flammable vapour sensor fault | Call a pro | $150–$700 |
| SENSEI | Exhaust gas temperature sensor fault | Call a pro | $150–$600 |
| SENSEI RX | Exhaust gas temperature sensor fault | Call a pro | $150–$600 |
Each row opens the full page for that generation. We hold manuals for Non-SENSEI, RH180 Hybrid, SENSEI and SENSEI RX, and each one gets its own reading of code 38 — what Rinnai documents, what you can safely check yourself, and what the repair should cost.
What each manual actually says
The table above compares 4 series in four columns, which is enough to route you and not enough to explain anything. This is the Rinnai wording behind each row, one entry per manual, so you can see where the 4 readings of code 38 agree and where they part company.
Non-SENSEI — Exhaust gas temperature sensor fault
The sensor in the exhaust port is out of range or not reporting. Rinnai checks the harness, then the sensor's resistance against the same curve used for the other thermistors, and then asks a question that matters on units this age: has this heater ever been converted from one gas type to another, and was the conversion done to specification? One warning if you own the RH180 hybrid rather than a tankless unit: on that appliance code 38 is a carbon monoxide sensor, not this one. The RH180 has its own page on this site — read that instead, because the difference is a safety device, not a wording preference.
RH180 Hybrid — Carbon monoxide or flammable vapour sensor fault
On the RH180, code 38 is the CO or FV sensor — the carbon monoxide sensor. This is the single most important difference between this appliance and the rest of the Rinnai range, where the same number is an exhaust gas temperature sensor. If you have arrived here from a page or a forum post about Rinnai code 38, that page is describing a different component on a different product. Rinnai's remedy is three lines — check the sensor wiring for damage, measure the sensor's resistance, replace the sensor.
SENSEI — Exhaust gas temperature sensor fault
The sensor in the exhaust port is not reporting a believable temperature. Rinnai works through the wiring, then a resistance check against the standard thermistor table, then the combustion side — gas type, vent clearances, parameter settings, manifold gas pressures — before the sensor is finally replaced. When Rinnai's checklist here reaches the combustion side, remember what this generation does not have: no dedicated lime sensor, so nothing is independently reporting scale except the SS warning. On the SENSEI RX the same investigation has code 34 and the LC lockout to lean on.
SENSEI RX — Exhaust gas temperature sensor fault
The sensor in the exhaust port is not reporting a believable temperature. Rinnai's sequence is the wiring first, then a resistance check against the standard thermistor table, then the combustion side — gas type, vent clearances, parameter settings, manifold gas pressures — before the sensor is finally replaced.
How much the answer changes between series
Non-SENSEI, RH180 Hybrid, SENSEI and SENSEI RX all route code 38 the same way: a professional, at $150–$700. That makes it one of the Rinnai numbers you can act on before working out which unit you own.
The spread is real money. On the Non-SENSEI the worst case is $600; on the RH180 Hybrid the same number can reach $700. That is a $100 gap behind one number.
Which Rinnai do you have?
On the 4 series that report code 38, the model number on the rating label starts C, RC, RU, RUC and RUR — match yours against the 78 listed below. You can also use the full model-number decoder.
Where this comes from
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 42), read Aug 2026 — the manufacturer's own documentation for this Rinnai range.
Source: Rinnai RH180 Operation and Installation Manual (100000319) (p. 36), read Sep 2026 — the manufacturer's own documentation for this Rinnai range.
Parts, guides and symptoms behind code 38
Code 38 on these 4 Rinnai series points at 3 parts and 2 symptoms, each named by the manufacturer's own procedure for this code.