Rheem Tankless Error Code 12: Flame Loss

The Short Answer: Rheem tankless 12 is flame failure after a successful light: the board proved flame, then lost it. That is not 11 (never lit) and not 10 (air/exhaust). Photograph it. Check the gas cock and the vent cap. Reset once. Repeats are the rod, wind, or pressure under load — licensed, not a reset loop.

Search Intent & The Real Problem

The remote or the PCB LED is flashing 12. The shower started, then died. This is Paloma-based Rheem gas tankless — RTG, RUTG, RTGH, Performance, EcoSense, Prestige condensing twins, and many Ruud/Richmond badges — not a tank Rheem with a status light. 11 never proved flame: flashing 11 / code 11. 12 had fire and dropped it. Same split as Rinnai 12 and Takagi 111 on sister boards.

A cold tank Rheem is not heating. Mid-shower dropout with no code is often min-flow — tankless goes hot to cold.

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 holds. Ten resets glaze the rod and flood the cabinet. They send homeowners at a manometer. They treat 12 like 11 (never lit) or like a packed inlet screen that never calls for fire. And they “fix” 12 by descaling first — scale is 14/16 until proven otherwise.

Step-by-Step Troubleshooting

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

• What to look for: Two digits on your remote or 7-seg LED before you kill power. Rating plate: RTG-64/84/95, RTGH, RUTG indoor, Paloma-badged twins. Electric Rheem tankless (RTE / RTEX) uses E-codes, not 12.
• Required tools: Phone photo of the code and the plate. Reset is last — same discipline as a Takagi reset.

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 unit that was fine in July. 1/2” flex on a 160–199 kBTU Rheem lights the range and drops this when the furnace joins. 12 only on the coldest mornings is undersized gas, not a bad rod. New meter relight can be air — licensed purge, not a union crack. No gas anywhere is not getting gas.
• 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).

Step 3: It did light — flame rod, wind, one trial

• What to look for: Cover on. One hot-water call. Fan, spark or glow, brief blue, then dark and 12. That is sense, wind, or pressure sag under load — not “it never had gas.” Rod sooted so ionization drops (licensed microamps). Loose flame-rod connector after someone “cleaned it.” Outdoor RTG on a windward wall slamming the flame out as the fan ramps. Rain in the cabinet: dead after rain. A bang is delayed ignition — loud bang — shut it down, not a second trial.
• Required tools: Ears and eyes. One trial. A light wipe on a cool rod is the most a homeowner should do. Do not sandpaper it. Do not bend the gap. Do not run it with the cover off “to see the flame.”

A 12 with a flame you can hear is almost never “no gas.” It is proof, wind, or the valve.

Step 4: Fan, vent, condensate — the 12 that is really air

• What to look for: Combustion fan silent on a call — stop. A dead blower is CO — blower not working. Outdoor cap packed with snow or a nest. Indoor Category III crushed or pulled apart. Condensing Prestige: condensate backing into the exchanger so the flame drowns — that often throws 10 first, but a 12 after a gurgle is the same water. Exhaust odor indoors is not a 12 you reset through — tankless exhaust smells like gas.
• Required tools: Eyes on the termination from the ground. Do not garden-hose the vent.

Inlet-screen grit causes no-fire without a 12 more often than it causes 12. A trickle that never calls is tankless pressure drop, not this code.

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

• What to look for: Cause addressed (cock open, LP in the bottle, cap clear, fan spinning). Remote ON/OFF off 10–30 seconds, on — or kill the appliance switch the same way. Unit fires, holds, no bang, no 12 on the next shower. Same 12 on the next call is the end of DIY.
• Required tools: Your hand on the switch. No paperclip in the PCB.

House static over ~80 psi and a dumping T&P (150 psi / 210°F typical) is pressure, not 12. Do not hold a tripped breaker On — breaker will not reset.

When DIY Ends: Calling a Licensed Pro

Stop at gas odor, delayed-ignition bangs, a 13, a 12 that returns, a silent fan, a wet board, and any gas-pressure or gas-valve work. You may photograph the code, confirm the shutoff, look at the termination, 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 tankless error code 12 mean? A: Flame failure after ignition. The burner lit, then the flame died or the flame rod stopped proving it. 11 is never lit. 10 is air or exhaust. Electric Rheem tankless units do not use code 12.

Q: How do I clear a Rheem tankless code 12? A: Photograph it, fix the obvious cause (gas cock, LP, vent cap, one GFCI), 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.”