Rheem Tankless Gas Code 35
Thermistor connected incorrectly. Not a failed sensor: the unit has four thermistors, and one or more has a poor connection or is plugged into the wrong location. It is a wiring fault, and it appears most often after someone has had the unit open.
Call a proThe verdict
Call a licensed plumber
Almost always a consequence of recent work. If the unit was serviced or had a board replaced, call that technician back — this code is pointing at their wiring, and it should not be a new diagnostic charge.
Typical cost: $0–$400
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 Tankless Gas — not averaged across the brand.
Source: Rheem Tankless Gas Troubleshooting Pocket Manual (SVC820) (p. 66), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Check whether the unit was serviced, opened, or had parts replaced recently. This code points straight at that work.
- Contact whoever did it and quote the code.
- Do not open the unit to move connectors yourself.
Frequently asked questions
Why would a thermistor be in the wrong place?
The four sensors are similar and their connectors reach more than one position. Swap two during a repair and each reports a plausible temperature from the wrong part of the unit — which is exactly what this code catches.
Don't confuse this with
4 other Rheem Tankless Gas codes point at a thermistor 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: