Equipment error code

Rinnai code 12

Code 12 — flame failure. Flame was sensed then lost, or could not be maintained during operation.

Equipment: water heater
Brand: Rinnai
Source checked: 2026-03-01

What this code means

Code 12 — flame failure. Flame was sensed then lost, or could not be maintained during operation.

Source: Rinnai Tankless Water Heater Operation Manual — Error Code 12 · checked 2026-03-01

Which models show it

Rinnai RL, RU, RUR, RX series tankless heaters.

What is actually happening

Water goes cold; display 12. May happen under high demand or after brief heat.

Is it safe to keep running?

Reduce demand until serviced if flame instability suspected.

Shut it down when

Gas smell, CO alarm, or code 12 on every draw.

What a homeowner can safely check

Check vent and air intake.
Clean air filter.
Avoid running multiple showers during test.
Power cycle once.

When to stop

Persistent 12 with gas odor.
Soot at vent cap.

Likely causes ranked

1. Flame rod cleaning or adjustment needed
How a tech confirms: Microamp test; set gap per Rinnai spec.
Typical repair: $170–$380 · Water Heater Replacement cost guide
2. Exhaust restriction or condensate backup
How a tech confirms: Draft test; clear condensate.
Typical repair: $185–$500 · Water Heater Replacement cost guide
3. Gas pressure drop under load
How a tech confirms: Manometer with fixtures flowing.
Typical repair: $210–$550 · Tankless Water Heater Conversion cost guide
4. Scale buildup causing overheat shutdown
How a tech confirms: Descale service; flow verification.
Typical repair: $260–$600 · Tankless Water Heater Conversion cost guide

Is this the end of this unit?

Exchanger failure after neglected descale — replacement unit often warranted.

How to clear the code

Power off/on; test single fixture low flow.

Why clearing is not fixing it: Flame must stay proven — intermittent faults recur.

Which trade to call

Call a licensed plumber who works on water heater equipment. Gas, refrigerant, and combustion faults are not general handyman jobs.

How to find a good plumber

Before you sign a contract for this job, use our guide on how to find a good plumber: verify the licence at the state board, compare three itemized quotes on the same scope, and put permit responsibility and change-order rules in writing.

Read the plumber hire guide Check if your quote is fair Verify licences by state

More codes from this brand

Code 11 — no ignition

Symptom guide: No Hot Water

FAQ

Rinnai code 12 meaning?

Flame failure — lost flame signal.

Code 12 only winter?

Check vent icing and wind — common seasonal trigger.

12 after code 11?

May indicate progressing ignition system issue — service both codes history.

Repair cost code 12?

$170–$600 typical.

Annual service help?

Rinnai recommends filter and vent checks to reduce 10/11/12 faults.

Source

Source — Rinnai Tankless Water Heater Operation Manual — Error Code 12. Checked 2026-03-01.