Equipment error code

Rheem code 12

Code 12 — flame failure. Flame was established then lost during operation, or failed to maintain during the run cycle.

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

What this code means

Code 12 — flame failure. Flame was established then lost during operation, or failed to maintain during the run cycle.

Source: Rheem Tankless Water Heater Use & Care Manual — Error Code Table (AP13155) · checked 2026-03-01

Which models show it

Rheem RTGH, RTG tankless gas models with flame rod proving.

What is actually happening

Water turns cold mid-use; display shows 12. Burner may have lit briefly then dropped.

Is it safe to keep running?

Limit hot water demand until serviced — flame-outs can indicate venting or gas issues.

Shut it down when

Gas odor, carbon monoxide alarm, or repeated code 12 on every draw.

What a homeowner can safely check

Check gas supply to other appliances.
Ensure vent and air intake clear.
Avoid simultaneous multiple high-flow fixtures during test.
Single power reset after checks.

When to stop

Code 12 with gas smell.
Visible soot at vent.

Likely causes ranked

1. Dirty or corroded flame rod
How a tech confirms: Microamp test; cleans or replaces rod.
Typical repair: $175–$360 · Water Heater Replacement cost guide
2. Intermittent gas pressure or valve issue
How a tech confirms: Inlet pressure under load; valve coil test.
Typical repair: $200–$525 · Water Heater Replacement cost guide
3. Vent backdraft or wind reversal
How a tech confirms: Combustion spillage test; adds wind guard or re-terminates vent.
Typical repair: $225–$600 · Water Heater Replacement cost guide
4. Heat exchanger scale causing overheat flame-out
How a tech confirms: Flow test and descale procedure per Rheem.
Typical repair: $250–$550 · Tankless Water Heater Conversion cost guide

Is this the end of this unit?

Heat exchanger failure on scaled tankless past warranty — replacement often beats exchanger swap.

How to clear the code

Power cycle once; run single low-flow hot tap to test.

Why clearing is not fixing it: Flame must prove continuously — intermittent faults return on next demand.

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

Symptom guide: No Hot Water

FAQ

Rheem code 12?

Flame failure — lost flame during operation.

Code 12 only on high flow?

Can indicate gas sizing or scale limiting output — tell your plumber flow rate when it trips.

Repair cost code 12?

$175–$600 typical; descale or exchanger higher.

DIY flame rod clean?

Possible with gas off per manual — many use licensed plumbers for gas appliances.

12 vs 1?

Code 1 is failed initial ignition; 12 is lost flame after lit.

Source

Source — Rheem Tankless Water Heater Use & Care Manual — Error Code Table (AP13155). Checked 2026-03-01.