Rheem Tankless Gas Code 90
Vent blockage detected before ignition. Rheem documents codes 90 and 99 together: the unit was operated before the vent was installed, or it detected a blockage in the venting during the purge cycle. Code 90 is the one that occurs **before** the unit goes to ignition — it refused to light because the vent path did not check out.
Call a proThe verdict
Call a licensed plumber
The unit refusing to light is the safe outcome. Check the vent terminal outside, perform the reset procedure once, and if it returns get the vent system inspected — do not keep resetting past it.
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 snow, leaves, a nest, or anything restricting it.
- Perform the reset procedure in your Rheem manual once — Rheem requires a reset to clear this code.
- Do not disconnect or take apart any vent section yourself.
- If it returns, book a licensed professional.
Frequently asked questions
What is a purge cycle?
Before lighting, and again after shutting down, the fan runs to clear the combustion chamber and the vent of any gas. It is also when the unit gets to measure whether air is moving as it should — which is how a blocked vent gets caught before ignition rather than after.