Rheem Tankless Error Code 12: Flame Failure — Meaning, Causes & Fix

Rheem tankless (RTGH / RTG / IKONIC): flame detected, then lost. The burner lit and the flame rod proved flame, but the flame dropped out during operation; Rheem's manual states the unit counts these flame-loss incidents and displays 12 after 3 consecutive losses. The RTGH service chart gives the plain-language causes: gas service interrupted, or LP gas running low.

Code 12 begins where code 11 ends. The ignition sequence succeeded — the flame rod proved a real flame — and the heater settled into heating. Then, mid-run, the flame signal vanished. Rheem's Tankless Troubleshooting Manual (AP21822) describes the behaviour exactly: if the water heater detects loss of flame during combustion, it stops supplying fuel, attempts to restart, and counts the incidents of flame loss; if the incident occurs 3 times consecutively, the unit displays 12 on the built-in controller or the remote. That counting matters for diagnosis: a single gust or a one-off pressure dip produces a restart you may never notice, while a 12 on the display means the flame is failing repeatedly and predictably. The causes sit in three groups. First, fuel delivery that degrades while running: LP gas running low (the RTGH chart names it directly), a gas service interruption, a gas filter at the heater's connection slowly strangling flow, or dynamic pressure sag when other appliances draw on the same line. Second, flame sensing that degrades: a flame rod coated in deposits may prove a strong flame at light-off yet drop the weaker or shifting signal mid-burn, and Rheem's diagnostic flow for 12 is built around cleaning and testing that rod. Third, combustion air: venting that is partly blocked or a termination exposed to gusts can disturb the flame until it lifts off the burner. The diagnostic check items Rheem lists for 12 are the same family as 11 — gas supply pressure (4.0-10.5 in. W.C. natural gas, 8.0-13.0 in. W.C. LP, checked dynamically), venting, gas filter, flame rod, and DIP switch settings — because the two codes share hardware and differ mainly in when the flame proof fails. A heater showing 12 usually still delivers some hot water in bursts between shutdowns, which is a useful symptom to report: cold bursts mid-shower with a 12 on the controller is the signature of flame loss under way, not a dead unit. Rheem Australia's chart compresses the same fault into four words — 'flame failure during operation' — and its owner-level remedies sit at the fuel end of the system, which matches field experience: most 12s are born in the supply or the sensing, not in the burner itself.

What causes the 12 code on a Rheem water heater?

LP gas running low, or gas service interrupted

Rheem's RTGH service chart reduces code 12 to two owner-level causes: gas service interrupted — contact the gas utility — or LP gas running low — refill or replace. A propane tank near empty delivers falling pressure exactly as demand rises, so the burner lights on what pressure exists and then starves. This is the first thing to check on any LP installation, and the cheapest: read the tank gauge before booking anyone. A tank showing a quarter or less in cold weather is effectively empty for a high-draw appliance, because the liquid vaporises more slowly as its temperature falls.

Dynamic gas pressure sag under load

A supply that is adequate at light-off can sag minutes later when a furnace, range, or dryer joins the draw. Rheem's manual insists on dynamic pressure testing at maximum combustion for exactly this reason, and specifies 4.0-10.5 in. W.C. (NG) and 8.0-13.0 in. W.C. (LP). The pattern to recognise: hot water fails during the morning peak or when the heating system runs, and behaves at quiet times. Undersized piping and tired regulators are the usual root causes, and both are licensed-gas work.

Gas filter at the gas connection partially blocked

The same inlet gas filter Rheem flags for code 11 matters even more for 12: a partial blockage passes enough gas to light the burner, then restricts flow as the modulating valve opens up, and the flame starves and drops. Debris from pipe work or an ageing line collects there. Because reaching it means opening a gas joint, it is a technician task, but it belongs on the list whenever 12 follows recent gas work or appears on an older installation that has never been serviced.

Flame rod fouled or failing

The flame rod proves flame by passing current through it; deposits on the rod weaken that signal. A marginal rod can pass the strong, stable flame of light-off and then lose the signal as the burner modulates, vibrates, or the flame shifts — which is precisely a flame 'loss' the control counts. Rheem's flow chart routes through reseating the rod's connector, removing and cleaning the rod, and replacing it if the code persists, before condemning the control board. This is the most common internal cause of 12 on a unit with a known-good gas supply.

Venting disturbance or blockage

Partly blocked venting, a disconnected section, or a termination where wind gusts across the exhaust can disturb combustion air enough to lift the flame off the burner mid-run. Rheem's check is the condition of the venting — connections and both terminations, inlet and exhaust sides. A 12 that correlates with windy weather or that began after roof or siding work deserves a termination inspection before any part is replaced. Gusty rooftop terminations and damaged concentric vent caps are repeat offenders, and both can often be spotted from the ground before anyone climbs anything.

Checks to run before you replace anything

1. If the home runs on LP/propane, read the tank gauge first — Rheem's own chart puts 'LP gas running low' at the top of this code's causes.
2. Note the pattern: does the hot water fail mid-shower or only when other gas appliances run? Timing is the strongest clue you can hand a technician.
3. Confirm the gas shut-off valve at the heater is fully open and other gas appliances run steadily, not weakly.
4. Inspect both vent terminations outside for blockage, damage, or anything newly built or planted in front of them.
5. Check the air inlet screen/filter on the unit for dust and lint loading.
6. Run one hot tap at a moderate flow and watch the controller: how long does the burner run before 12 appears — seconds (sensing) or minutes (supply sag)?
7. Make one documented reset attempt after these checks; if 12 returns the same way, the pattern — not the reset — is your answer.

How to fix rheem water heater error code 12?

1. Refill or replace the LP tank if the gauge is low, or confirm with the gas utility that service is not interrupted.
2. Open the heater's gas shut-off valve fully and clear any blockage at the vent terminations; reconnect any slipped vent section.
3. Clean the air inlet screen/filter.
4. Reset once using the documented procedure: close the hot faucet, turn the controller off, switch off the electrical supply, wait 5 minutes, restore power, turn the controller on, and open a hot tap.
5. If 12 returns, book a licensed technician for the internal sequence Rheem specifies: dynamic pressure test, gas filter inspection, flame rod cleaning/testing, then gas valve and control board checks.
6. Stop using workarounds — a heater losing flame repeatedly is shutting down for a reason; do not prop vents open or defeat the flame proving circuit.

The part that usually fails: Gas filter (at the gas connection)

Tucked into the gas connection at the bottom of the heater is a small filter screen whose whole job is to catch debris before it reaches the gas valve. Rheem's manual names it a diagnostic check item for both 11 and 12. A clean filter passes full flow at any firing rate; a loaded one behaves like a kinked hose — enough gas to light, not enough to sustain a modulated flame — which is why it produces flame loss rather than a clean no-ignition. Cleaning or replacing it requires breaking and re-making a gas joint with leak testing afterwards, so it is firmly technician work, but asking whether it was checked is a fair question to put to anyone diagnosing a stubborn 12.

Rheem tankless water heater error code 12 reset

Use the same documented Rheem procedure as for other tankless fault codes: close the hot water faucet, turn the remote or built-in controller off, switch off the electrical supply to the unit, wait 5 minutes, then restore power, turn the controller back on, and open a hot water faucet. On IKONIC models, alarms can also be cleared from the display with ENTER and answering Yes to 'Clear Alarms?'. Resetting clears the flame-loss counter, so a heater with a marginal fault may run for a while before 12 reappears — that delay does not mean it fixed itself. If 12 logs again after one reset with a known-full LP tank and clear vents, the remaining causes (pressure sag, gas filter, flame rod) all need tools and a technician.

Each Rheem platform resets the way its manual documents, and only that way. Tankless models: close the hot faucet, turn the controller off, switch off the electrical supply, wait 5 minutes, restore power, turn the controller on, and open a hot tap; IKONIC displays also clear alarms via ENTER and 'Clear Alarms?' — Yes; hard lockouts (10, 13, 99) have a DIP-switch procedure that ends with the remote flashing CL (fault history cleared) or UL (reset confirmed). Tank gas controls have no reset button: the status LED clears when the underlying fault clears, and Rheem's chart routes 4, 5, 7 and 8 flashes straight to a qualified technician. Hybrids and smart electric tanks: read and note the alarm first (Next / Down arrows on the hybrid display), fix the cause, then clear; a leak alarm such as A107 can be overridden for 24 hours only by holding Clear Alarm for 5 seconds — time bought to arrange the repair, not to skip it. One reset after fixing the cause is the documented pattern on every platform; repeated resets against a live fault are not a strategy.

Why does the 12 code keep coming back?

Code 12 is a pattern code, so it comes back in patterns. Fill the LP tank and it vanishes until the tank runs low again. Clean the flame rod without fixing a pressure sag and it returns on the first cold morning the furnace runs hard. Clear a vent termination and it returns when the nest is rebuilt. The count-to-three behaviour also means the fault can simmer invisibly — single flame losses, silently restarted — before it ever displays. If your household notices unexplained cold bursts in the shower, treat that as early 12 behaviour and get the supply and rod checked before the lockouts start.

How to stop 12 coming back

• LP gas running low, or gas service interrupted: Rheem's RTGH service chart reduces code 12 to two owner-level causes: gas service interrupted — contact the gas utility — or LP gas running low — refill or replace. A propane tank near empty delivers falling pressure exactly as demand rises, so the burner lights on what pressure exists and then starves. This is the first thing to check on any LP installation, and the cheapest: read the tank gauge before booking anyone. A tank showing a quarter or less in cold weather is effectively empty for a high-draw appliance, because the liquid vaporises more slowly as its temperature falls.
• Dynamic gas pressure sag under load: A supply that is adequate at light-off can sag minutes later when a furnace, range, or dryer joins the draw. Rheem's manual insists on dynamic pressure testing at maximum combustion for exactly this reason, and specifies 4.0-10.5 in. W.C. (NG) and 8.0-13.0 in. W.C. (LP). The pattern to recognise: hot water fails during the morning peak or when the heating system runs, and behaves at quiet times. Undersized piping and tired regulators are the usual root causes, and both are licensed-gas work.
• Gas filter at the gas connection partially blocked: The same inlet gas filter Rheem flags for code 11 matters even more for 12: a partial blockage passes enough gas to light the burner, then restricts flow as the modulating valve opens up, and the flame starves and drops. Debris from pipe work or an ageing line collects there. Because reaching it means opening a gas joint, it is a technician task, but it belongs on the list whenever 12 follows recent gas work or appears on an older installation that has never been serviced.

Retrieving stored fault codes on Rheem water heaters

Rheem water heaters report faults in three separate languages, and reading the right one is the whole skill. A tank gas heater with an electronic combination gas control has no screen: a single status LED on the control blinks in patterns — 1 flash for normal operation, and 2, 4, 5, 7 or 8 red flashes for faults (Rheem's chart deliberately has no 3- or 6-flash entries). Power-vent models on the IntelliVent control instead use lettered LEDs (A-F) as temperature indicators and error displays. Gas tankless models (RTGH, RTG, IKONIC) show two-digit numeric codes on a built-in display or remote — 11 for no flame detected, 12 for flame lost, P1 for low flow, 1L for scale — and IKONIC units keep a scrollable fault history of recent faults. Hybrid heat-pump and EcoNet smart electric models speak in alphanumeric alarms: press Next until 'Current Alarm' appears, then scroll with the Down arrow; A-codes are shutdown alarms, T-codes are alerts. Electric tankless models on the RTEX/EcoSmart platform use short E-codes (E1-E5). Always identify the platform before looking a code up: the same characters mean different things in different languages.

Other Rheem water heater error codes

See the full list on the Rheem Water Heater error codes hub.

Codes 12 gets confused with

11 — Rheem tankless (RTGH / RTG / IKONIC): flame not detected by the flame rod. At ignition the unit tries to light twice, and if the flame rod never proves a flame, it shuts down and displays 11. The RTGH service chart gives the classic owner-level cause: the gas shut-off valve is not fully opened. A Rheem tankless heater lights in a strict sequence.

29 — Rheem tankless, and the meaning depends on generation. On condensing IKONIC-era models: condensation is not draining from the collector — the built-in drain trap's electrode reports the trap full or unreadable. On RTGH condensing models the service chart says the neutralizer is clogged. On older RTG and Rheem Australia models, 29 meant heat exchanger outlet temperature too low. Identify your model before acting. A condensing tankless heater squeezes extra heat from its exhaust, and the price of that efficiency is water: vapour in the flue gas condenses inside the secondary heat exchanger and must drain away continuously while the burner runs.

E5 — Rheem electric tankless (EcoSmart / RTEX platform): high flow rate. The water is moving through the heater faster than its elements can bring it to the set temperature, so the outlet runs lukewarm and the control reports E5. EcoSmart's troubleshooting guide — EcoSmart is Rheem's electric tankless division — lists E5 simply as 'high flow rate': a capacity report, not a failed part. An electric tankless heater has no stored hot water.

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Facts on this page were checked against manufacturer documentation: files.myrheem.com · files.myrheem.com · www.manualshelf.com · www.manualshelf.com · www.manualshelf.com · mans.io · manuals.plus · manuals.plus · manuals.plus