Water Heater Codes

Rheem 12: Flame Loss During Operation

Water was coming out hot a minute ago, then it wasn't — and now your Rheem tankless display reads 12. That's a flame-loss fault: the burner lit successfully, but something interrupted the flame signal partway through the draw.

Applies to: Rheem/Ruud tankless gas water heaters

What it means

The unit detected flame during ignition, then lost it during operation. Because the flame rod has to keep confirming a stable flame for the entire time the burner runs, anything that disturbs that signal even briefly — a pressure dip, a draft down the vent, a fouled sensor — is enough to trip this code mid-shower, well after the unit already proved it could light.

Most common causes

What you can safely check yourself

Two quick checks before calling anyone:

  1. Check gas supply is steady and fully on
  2. Look for obvious venting blockage

When to call a professional

Flame loss keeps recurring — needs professional flame rod/gas inspection.

Gas supply lines, gas control valves, and venting should only be worked on by a licensed plumber or HVAC technician. Mains/line-voltage electrical work should be done with the breaker off and, beyond a simple reset, by a qualified electrician or plumber.

What the fix costs

Flame-rod cleaning or replacement on a tankless unit is usually a $150–$400 repair including the service call, per HomeGuide's and Angi's water heater repair pricing. If venting turns out to be the culprit instead, correcting a vent run runs higher depending on how much pipe needs to be redone — get a firm number before agreeing to any venting work. These are national averages, not a quote; a licensed plumber needs to see the actual installation.

Related codes

All Rheem codes

Every Rheem water heater fault code — the full list on one page.

Safety. This code lookup tells you what the control board detected, not that a fix is safe to attempt. If you smell gas, see scorching, or a safety device (thermal fuse, ECO, flammable-vapor sensor) trips more than once, stop and call a licensed professional — do not keep resetting it.

Sources

This site is an independent reference, not a substitute for a licensed plumber or electrician. Gas water heaters can leak carbon monoxide, leak gas, or catch fire if serviced incorrectly. Anything involving the gas line, gas control valve, venting, or main electrical wiring should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.