Rinnai Non-SENSEI Code 38
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.
Call a proThe verdict
Call a licensed plumber
The part itself is inexpensive, but it sits in the exhaust stream and the diagnosis runs into gas type and combustion. Professional work.
Typical cost: $150–$600
US estimate for parts and labor. Prices vary by region and by who does the work — see how we estimate costs.
What the manual says
The meaning above is transcribed from the manufacturer's diagnostic documentation for the Non-SENSEI — not averaged across the brand.
Source: Rinnai Troubleshooting Tankless Water Heater Diagnostic Codes (100000652) (p. 42), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
Before anything else: if you smell gas, stop here, leave, and call your gas utility.
- Find out whether the unit was ever converted between natural gas and propane, and tell the technician — Rinnai's checklist asks it directly.
- Book a licensed professional. The sensor is in the exhaust port and the checks past it are combustion checks.
Frequently asked questions
I have an RH180. Is this my code 38?
No. Read the RH180 page for code 38 instead — on that appliance Rinnai's own manual heads the code "CO or FV Sensor", meaning the carbon monoxide sensor, and the answer is to stop using the unit and get a professional. A CO sensor fault is not something to work through from a page written for a different product.
Code 38 on other Rinnai series
The same code number can mean something different on another series — make sure you're reading the page for your exact unit:
Don't confuse this with
One other Rinnai Non-SENSEI code points at a sensor too. They are different components in different places, and ordering a part against the wrong one is the most common way this fault gets expensive: