Rheem Code 12: Flame Lost After Ignition

The Short Answer: Rheem code 12 on a Paloma-based RTG / RUTG / Prestige tankless is flame failure after ignition: the board proved fire, then lost it. That is not 11 (never lit) and not a gas-tank LED count. Photograph it. Confirm the cock and LP. Reset once. Do not measure manifold pressure yourself.

Search Intent & The Real Problem

The remote or the PCB LED says 12. The shower started, then died. Google mixed a Rinnai 12, a Honeywell flash count, and an electric 5-flash into one stew. On Rheem, Ruud, and Richmond tankless (Paloma PH / RTG-64 / RTG-84 / RTG-95 / indoor RUTG / many Prestige condensing boxes), 12 is flame lost after a successful light. 11 never got that far — Rheem code 11, flashing 11. 10 is air/exhaust. A cold Rheem tank with a status light on the gas valve is a different machine.

What other guides get wrong

They paste the entire 10/11/12/14/16 table into a 12 article and tell you to reset until it fires. Ten resets glaze the igniter and flood the chamber. They send homeowners at a manometer. They treat a Rheem tank 7-flash FVIR as “code 12.” And they descale a 12 first — scale is 14/16 until proven otherwise. Sister ignition lockout on another Paloma-class board: Takagi 111.

Step-by-Step Troubleshooting

Step 1: Photograph 12 vs 11 vs 10 — then stop guessing

• What to look for: Digits before you kill power. Rating plate: RTG, RUTG, RTGH, Prestige tankless, Ruud equivalent. Indoor Category III vs outdoor rain cap. A bang on a late light is delayed ignition — loud bang — shut it down, not a second trial.
• Required tools: Phone photo of the code and the plate. No wrenches.

If there is no code and it simply passes cold water, you may have no flow call, not a 12. Packed inlet screen and closed isolation valves never ask the burner to stay lit.

Step 2: Gas cock, other appliances, LP — no manometer

• What to look for: Appliance shutoff handle parallel to the pipe (open). Stove and furnace lighting. LP bottles not empty; a first-stage regulator frosting on a 20°F morning will 12 a Rheem that was fine in July. 1/2” CSST or flex on a 180–199 kBTU chassis is a starvation install — the range lights, this burner lights, then drops when the furnace joins. 12 only on the coldest mornings is undersized gas, not a bad rod. New meter relight in the last day can be air in the line. That is a licensed purge and leak test, not a union crack. No gas at other appliances is not getting gas, not a board.
• Required tools: Eyes on the cock and the flex. No fittings wrenches.

Inlet and manifold pressure are licensed (NG often ~3.5–7 in. w.c. / LP ~10–11 in. w.c. — confirm the rating plate, never by ear). Fuel DIP or orifice set to the wrong gas after a move is a licensed correction.

Step 3: Flame rod, wind, and one clean trial

• What to look for: Cover on. One hot-water call. Listen: fan, spark or glow, blue, then silence and 12. That is sense, wind, or pressure under load — not “no gas.” Spark with a dark burner is 11 territory, not this page. Brief yellow puff then 12: delayed light or a wet/sooted burner after rain in the cap. Do not run it with the cover off “to see better.” A light wipe on a cool flame rod is the most a homeowner should do. Do not sandpaper it into a needle. Microamp flame current is a licensed reading.
• Required tools: Ears and eyes. One trial.

Repeated 12 with a wet cabinet is a leak first — Rheem leaking inside. Condensate drowning the flame often throws 10 first; a 12 after a gurgle is the same water.

Step 4: Vent, fan, and condensate that starve a running fire

• What to look for: Outdoor RTG on a windward wall, missing recirc cap, snow over the flue. Wind slamming the flame out the moment the fan ramps. Indoor RUTG: termination too close to a soffit, long Category III run, or a condensing Prestige with a condensate trap full so the fan never holds air. Bird nest, dryer blowing into the same corner, vinyl intake mashed shut. If the combustion fan is silent on a call, stop. A dead blower is carbon monoxide if anything burns — blower not working.
• Required tools: Eyes on the cap from the ground. Do not garden-hose the vent.

Clear the exterior. Do not foil a vent joint. A power-vent tank with a dead blower is not a Rheem 12.

Step 5: One reset after the cause — then hire it

• What to look for: Cause addressed (cock open, cap clear, LP not frozen). Power off 10–30 seconds, on. Unit fires, holds flame, no 12 on the next shower. Same 12 on the next call is the end of DIY. Inlet screen grit causes no-fire without a 12 more often than it causes 12; do not descale a 12 as step one. Do not hold a tripped breaker On — breaker will not reset.
• Required tools: Your hand on the switch. No paperclip in the PCB.

When DIY Ends: Calling a Licensed Pro

Stop at gas odor, delayed-ignition bangs, a 12 that returns after one reset, a silent fan, a wet board, remaining-flame 13, and any gas-pressure or gas-valve work. You may photograph the code, confirm the shutoff, look at the termination, rinse an inlet screen with water isolated, and power-cycle once. Combustion, manometer, flame-rod replacement inside the burner, and PCB work are licensed. Rheem will not smile on a rod you reset through a flooded burner all weekend.

Frequently Asked Questions

Q: What does Rheem water heater code 12 mean? A: On Rheem/Ruud tankless, 12 is flame failure — it lit, then lost flame or the rod stopped proving it. 11 is never lit. 10 is air/exhaust. A gas-tank LED flash count is a different chart.

Q: How do I reset a Rheem code 12? A: Photograph the code, fix the obvious cause (gas cock, LP, wind at the cap), then power off 10–30 seconds. One reset. A 12 that returns is a licensed diagnosis, not a reset schedule.

Q: Can I check gas pressure on a Rheem 12 myself? A: No. Confirm the shutoff and that other appliances light. Inlet and manifold pressure are a licensed manometer job. Do not loosen fittings to “bleed air.”