Rheem Tankless Gas Code 13
Poor combustion detected (indoor units). The second of Rheem's indoor-only codes: the flame rod is detecting poor combustion. Rheem names the common causes as inadequate gas supply and/or venting.
Call a proThe verdict
Call a licensed plumber
Poor combustion on an indoor gas appliance is the condition that produces carbon monoxide. Stop using it, make sure you have a working CO alarm, and get the gas supply and vent system checked properly — this is not a code to reset past.
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. 48), 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.
- Stop using the unit and leave it off.
- Check that your carbon monoxide alarms work. If you have none, fit one before the unit runs again.
- If you smell gas, leave the building and call your gas utility from outside.
- Book a licensed professional.
Frequently asked questions
Is code 13 dangerous?
It reports the condition that generates carbon monoxide, on a unit installed inside a building. The heater flagging it is the system working; continuing to run it while it flags is the risk.
Don't confuse this with
One other Rheem Tankless Gas code points at combustion itself too. They are the same symptom reported from a different place in the unit, so which one the display shows is what tells you where to look: