Rheem Tankless Gas Code 31
Inlet thermistor malfunction. The sensor reading incoming cold water temperature is not reporting correctly. It is the reference the unit uses to work out how much heat it needs to add, so a bad reading affects everything downstream of it.
Part replacementThe verdict
Replace a part
An inexpensive sensor with a service call around it. Rheem's own diagnosis is a resistance measurement at the board, which decides quickly between a failed thermistor and a wiring problem.
Typical cost: $150–$450
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. 58), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note whether the water temperature has been inconsistent at the tap.
- Note when the unit was last flushed — scale on a water-side sensor is a common cause.
- Book a service call.
Frequently asked questions
Rheem's table has four thermistor codes. Which is which?
31 inlet, 32 heat exchanger, 33 outlet, 34 ambient — and 35 for a thermistor that is connected wrongly rather than broken. Four sensors, four codes, plus one for getting them mixed up.
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: