Rheem Tankless Gas Code 51
Gas control valve: flame present after shutdown. The unit detected the presence of flame after the demand for hot water ended and it had turned off. Rheem attributes it to a gas control valve malfunction — meaning gas is still reaching the burner when it should not be.
Call a proThe verdict
Call a licensed plumber
A gas valve that may not be closing fully is the most serious class of fault on a gas appliance. Shut the manual gas valve below the unit and get it looked at — do not reset it and carry on.
Typical cost: $400–$1,100
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. 67), 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.
- Turn the heater off and close the manual gas valve below it.
- If you smell gas, leave the building and call your gas utility from outside.
- Book a licensed professional now rather than at the weekend.
Frequently asked questions
Is this the same as code 80?
Rheem gives both the same explanation — flame detected when demand ended and the unit turned off — and both point at the gas control valve. Treat either with the same seriousness.
Could it be the flame rod misreading instead?
It can be, and a technician will establish which. That is a reason to have it diagnosed properly, not a reason to assume the cheaper answer.
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: