Rinnai SENSEI Code 38
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.
Call a proThe verdict
Call a licensed plumber
The part is cheap; where it lives is what makes this professional. The sensor sits in the exhaust path, and Rinnai's troubleshooting escalates from it to gas pressures and venting.
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 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.
- Check the vent terminal outside for obstructions — restricted exhaust is on Rinnai's list for this code.
- Note any recent gas conversion or parameter change.
- Leave the unit off and book a licensed professional.
Frequently asked questions
How is code 38 different from code 54?
Code 38 says the exhaust temperature sensor is faulty. Code 54 says the sensor is working and reporting an exhaust temperature that is genuinely too high. Same sensor, opposite conclusions.
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
2 other Rinnai SENSEI codes point 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: