Rheem Tankless Gas Code 72
Flame detected before ignition. The flame rods are detecting the presence of flame before the igniter has even been activated. Either there is flame when there should not be, or the sensing circuit is reporting one — and the unit refuses to start either way.
Call a proThe verdict
Call a licensed plumber
The unit is refusing to ignite into a condition it does not understand, which is the correct behaviour. Do not work around it. The usual finding is the sensing circuit rather than a real flame, but that determination is a technician's.
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. 81), 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.
Frequently asked questions
Could this be a grounding problem?
It is a reasonable suspicion. Flame sensing works by passing a small current through the flame to ground, and a poor ground can make the board read current that is not really flame — the same mechanism behind false-flame codes on other brands.
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: