Rheem Tankless Gas Code 12
Flame lost after ignition. The unit detected the presence of flame and then lost it. As with code 11, Rheem names gas supply and venting as the common causes and points at the altitude DIP switch setting.
Call a proThe verdict
Call a licensed plumber
A one-off after a gas outage or a propane refill can be nothing. A repeating code 12 means the flame is genuinely dropping out, and settling that needs gas pressure measured under load and the combustion side inspected.
Typical cost: $150–$900
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. 46), 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.
- If you smell gas, leave and call your gas utility from outside.
- Confirm gas is on at the meter or tank and other gas appliances work.
- Note whether the code appears when another gas appliance fires — that pattern points at gas supply sizing.
- Book a licensed professional if it repeats.
Frequently asked questions
It only happens when the furnace runs.
That is close to a diagnosis in itself. It means total gas demand is pulling inlet pressure below what the water heater needs — a gas supply sizing problem rather than a water heater fault.
Don't confuse this with
4 other Rheem Tankless Gas codes point at flame detection 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: