Rheem Tankless Gas Code 99
Vent blockage detected after shutdown. The other half of Rheem's 90-and-99 pair: the same vent blockage detection, but occurring **after** the unit shuts down, during the post-purge cycle. Like code 90 it requires a reset before the unit will run again.
Call a proThe verdict
Call a licensed plumber
Same cause and same seriousness as code 90, caught at the other end of the burn. Check the vent terminal, reset once, and get the vent system inspected if it comes back.
Typical cost: $0–$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. 94), 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.
- Go outside and check the vent terminal for obstructions.
- Perform the reset procedure in your Rheem manual once.
- Do not disconnect or take apart any vent section yourself.
- If it returns, book a licensed professional.
Frequently asked questions
Why does it matter whether the blockage was found before or after?
For you, not much — the vent needs checking either way. For a technician it narrows things: a restriction that only shows after a burn behaves differently from one present from the start.