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.