Rheem Tankless Gas Code 05
Improper burner combustion (indoor units). A warning code, and one of two in Rheem's table that apply to indoor units only: the flame rod has detected improper burner combustion. Rheem names the usual causes directly — venting and/or gas supply.
Call a proThe verdict
Call a licensed plumber
A combustion quality warning on an indoor gas appliance is not something to sit on, even though the unit keeps running. Rheem points at venting and gas supply, both of which need instruments and both of which are how carbon monoxide problems start.
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. 35), 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 the building and call your gas utility from outside.
- Check that your carbon monoxide alarms work. If you do not have one near the unit, fit one.
- Look at the vent terminal outside for obstructions.
- Book a licensed professional rather than waiting for it to become an error code.
Frequently asked questions
Why is this indoor-only?
Because the consequences differ. An outdoor unit's combustion products disperse; an indoor one's go through a vent system into your house if that system is not right, which is why Rheem watches combustion quality more closely there.