Rinnai 12: Flame Failure

Hot water was running fine, then it stopped and the remote is now showing 12 — that's Rinnai's flame-failure code, meaning the burner lit successfully but lost its flame partway through the cycle.

Applies to: Rinnai tankless and condensing water heaters.

What it means

Flame Failure — the burner ignited but the flame was lost or the flame rod stopped sensing it during operation. The control board treats a missing flame signal as unsafe and cuts gas to the burner immediately rather than continuing to run without a confirmed flame.

Most common causes

A handful of conditions can interrupt a flame that was already burning:

Low or fluctuating gas pressure
Obstruction in the flue outlet or venting
Loose power supply connections or voltage drops
Flame rod disconnected or coated with carbon build-up
Condensation or debris on the burner surface

What you can safely check yourself

Before assuming the worst, there are a few things worth a look:

Check that gas is on at both the water heater and the meter
Visually check outdoor vent terminals for obvious blockage (leaves, snow, nests)
Power-cycle the unit

When to call a professional

The code returns after a power cycle and basic gas/vent checks — the flame rod, wiring, or gas valve likely needs professional inspection.

Gas supply lines, gas control valves, and venting should only be worked on by a licensed plumber or HVAC technician.

What the fix costs

A service call to diagnose flame loss typically runs $150–$600, with cleaning or replacing a fouled flame rod usually landing toward the lower half of that range (Angi, Fixr water heater repair cost guides). If unstable gas pressure turns out to be the underlying cause, correcting it may involve the gas utility or a regulator adjustment rather than a simple parts swap, which can push the bill higher. These are national averages, not a quote for your home — a local licensed plumber can give you an actual number once it's diagnosed.

Related codes

All Rinnai codes

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

Safety. If you smell gas, hear popping, or see soot near the burner, shut off the gas supply and call a licensed technician right away. A flame-loss fault that keeps returning after basic checks is not something to keep resetting on your own.

Sources

Rinnai tankless water heater service and error code documentation (manufacturer)

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.