Rheem Tankless Gas Code 66
Water by-pass valve malfunction. The bypass valve routes some water around the heat exchanger and blends it back in, which is how the unit holds a steady outlet temperature when demand changes. This code says it is not doing that reliably.
Part replacementThe verdict
Replace a part
Same class of repair as code 65 — a valve in the water path with a service call around it. Expect temperature that hunts up and down until it is replaced.
Typical cost: $200–$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 Tankless Gas — not averaged across the brand.
Source: Rheem Tankless Gas Troubleshooting Pocket Manual (SVC820) (p. 78), read Aug 2026 — the manufacturer's own documentation for this exact series.
What you can safely check
- Note whether the temperature hunts up and down during a shower.
- Book a service call.
Frequently asked questions
Could a failing bypass valve give me a cold water sandwich?
It can contribute, though most cold water sandwiches are plumbing rather than a failed valve. If yours began at the same time as this code, they are probably the same story.
Don't confuse this with
5 other Rheem Tankless Gas codes point at a valve 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: