Rinnai SENSEI RX Code 38
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.
Call a proThe verdict
Call a licensed plumber
The part is cheap; what makes this a professional job is where it lives. The sensor sits in the exhaust path, and Rinnai's own troubleshooting escalates from it to gas pressures and venting. Nothing here is safe to open up at home.
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 RX — 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 itself is faulty. Code 54 says the sensor is working and reporting an exhaust temperature that is genuinely too high. Same sensor, opposite conclusions.