Rheem Tankless Gas Code 33
Outlet thermistor malfunction. The sensor reading the temperature of water leaving the unit is not reporting correctly — the main feedback the heater uses to hold your set point. Rheem's check is a resistance measurement at connector R.
Part replacementThe verdict
Replace a part
Cheap part, service call around it. Until it is fixed the unit is controlling temperature without its most important reading, so expect wandering water temperature.
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. 62), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note how the temperature behaves at the tap — useful detail for the technician.
- Note when the unit was last flushed.
- Book a service call.
Frequently asked questions
Could scale cause this?
Yes. Any sensor sitting in the water path collects the same deposit that coats the heat exchanger, and a coated sensor reads slow and wrong before it reads as failed.
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: