Rheem Furnace Flame Error Codes (12, 13, 14): Meaning & Fixes

Three Rheem codes report on one system — the flame sense circuit that proves fire to the control: 12, low flame sense, where the signal is weak but heating continues normally; 13, flame lost, where a proved flame disappears mid-call while the thermostat is still asking for heat; and 14, unexpected flame, where flame is sensed with the gas valve supposed to be off and the furnace will not operate at all. This guide gives each code its own section in Rheem's wording, then the shared rod, wiring and ground story behind all three — and the escalation arithmetic, five losses to a lockout, that turns small flame faults into the one-hour lockout.

How Rheem proves flame: the system all three codes share

A Rheem furnace never takes its own word for fire. After the igniter lights the burners, a separate device has to confirm the flame exists and keep confirming it for as long as gas flows: the flame sense rod, a metal rod standing in the burner flame, wired back to the furnace control (I.F.C.). Flame passing over the rod produces the flame sense current the control watches, and every code in this article is that watch reporting a different failure of the same proof. Code 12 says the proof arrived but weakly — the current from the rod is marginal. Code 13 says the proof arrived properly and then stopped, while the heat call was still asking for fire. Code 14 says the proof is present at a time when there should be no flame at all, because the gas valve is supposed to be off. Three codes, one circuit, three moments: weak, lost, and impossible. Two properties of the system explain most of its behaviour. First, the signal is small and electrical, so its enemies are electrical as much as fiery: a coated rod, a loose or damaged connection, wiring shorted or open, a sensor improperly mounted, and a poor furnace ground all weaken or corrupt the proof without the flame itself changing at all — Rheem's cause lists for this family name the rod's condition, its connections and the furnace's ground before they name anything about the burners. Second, the control grades its responses by how dangerous the report is: a weak signal (12) is classed non-critical and heating continues normally, because flame is still being proved; a lost flame (13) ends that burner run, because gas must never flow unproved; and an unexpected flame (14) is, in Rheem's words, extremely critical, because flame with the valve supposedly off means either fuel is going somewhere it was not sent, or the control can no longer trust its own senses. Everything below is those three grades worked out code by code — and the discipline they share: the rod and its circuit are proved before the flame itself is blamed.

Code 12: low flame sense — the early warning

Rheem's definition is precise about both the fault and its weight: the flame sense current from the flame rod is weak or marginal, Rheem classes the code non-critical, and heating continues normally. A furnace can run for days or weeks showing 12 between its operating displays, heating the house apparently without complaint — which is exactly what makes 12 valuable and exactly why it gets ignored. Treat it instead as the first chapter of the other two codes' story. Rheem's cause list for 12 is short and rod-centred: most often the rod needs cleaning or is not properly connected — including wiring shorted or open — and also an improperly mounted sensor or a poorly grounded one. The rod lives in the flame, and what flame deposits on it season after season insulates it electrically a little at a time, so the current the control receives fades gradually rather than failing at once; 12 is the control announcing that the fade has reached its warning line. The connection and ground branches fade the signal the same way from the other end: a connector losing its grip, a wire chafing toward a short, a ground path corroded or disturbed by other work. Rheem's stated fix is equally direct — clean or replace the rod and check connections and furnace ground — with an assignment that matters: Rheem assigns sensor cleaning to a qualified installer or service agency, so rod work is technician work in Rheem's own framing, not an owner chore with a scouring pad. The reason to book that visit while the furnace still heats is in the escalation section below: left alone, a marginal signal does not stay marginal. The same rod that whispers 12 in October is the rod that loses flame outright in January, and 12 is the only member of this family that warns you while the repair is still cheap, scheduled and warm.

Code 13: flame lost — proof that arrived, then vanished

Code 13 is defined by its sequence, and Rheem's wording preserves it: after a successful ignition the flame was properly sensed, and then was no longer sensed, while a valid heat call is still present. Every clause excludes a neighbour. Because ignition succeeded and flame was properly sensed, 13 is not a failure to light — that is code 11's territory, where flame is never proved at all. Because the heat call is still present, 13 is not a normal shutdown at the thermostat's command. What remains is a flame proof that collapsed mid-run, and Rheem gives it two cause families. The first is code 12's family, matured: the rod needs cleaning, or its connection or the furnace ground has degraded past the point where a proved flame can stay proved — a furnace that showed 12s before its 13s has been narrating this progression in order. The second family is the flame itself becoming unstable: Rheem names an unstable flame pattern, and its fix branch sends a technician to the burner fit and the burner-door gasket and seals — the hardware that decides whether the flame sits steadily on the burners, licking the rod evenly, or lifts, flutters and wanders off it when the blower moves air through the cabinet. The two families feel different in the field and the difference is worth reporting: a rod-family 13 tends to arrive at a consistent point in the cycle and worsen steadily, while a flame-stability 13 often correlates with the blower starting, door or panel disturbances, or windy conditions working on the vent. Either way, the control's response is fixed and non-negotiable: gas stops when proof stops. A single 13 is a report; repeated 13s inside one heat call are counted, and the count has a destination — five flame losses in one heat call escalate to the one-hour lockout, displayed as 10 alternating with 13, which the reset guide on this site covers in full.

Code 14: unexpected flame — the code that stops everything

Code 14 inverts the family's logic: flame is sensed while the gas valve is supposed to be off. Rheem's grading is the strongest in this article — extremely critical — and its operational consequence is total: the furnace will not operate. Sit with the logic for a moment, because it explains the severity. The control commands no gas, yet its flame proof insists fire exists. Only three stories can produce that report, and Rheem lists all three as 14's causes. The wiring story: 24VAC field-miswired to the gas valve solenoid, so the valve receives power the control never sent — the flame is real, and it is being fed by an electrical mistake. The valve story: a gas valve stuck open, passing fuel after its command ended — again the flame is real, and the device whose whole job is to stop fuel has failed at it. The control story: a faulty control sensing flame that is not there — no fire at all, but a control whose flame proof can no longer be trusted, which for a device that meters gas is a failure of the same rank. Because two of the three stories involve real, uncommanded gas flow, the owner's response to 14 is not diagnosis but shutdown: do not run the furnace, do not keep resetting it to see if the code clears, and do not treat a 14 that disappears as a 14 that healed — wiring faults and sticking valves are intermittent precisely when they are most dangerous. Rheem completes the picture with the lockout that follows: once the unexpected-flame condition clears, a one-hour lockout (10 + 14) follows, and the furnace sits out its hour even though the alarming report has ended. The repair paths are licensed work end to end in Rheem's framing — wiring correction, gas valve or control replacement — and a furnace that has shown 14 stays off until one of those repairs is proven, not until the display forgives it.

Reading the three codes together: the escalation ladder

Individually, 12, 13 and 14 are three reports; together they form a ladder, and knowing its rungs changes what you do at each one. The bottom rung is weakness: 12, non-critical, heating continues, the rod or its circuit fading. The middle rung is loss: 13, the proof collapsing mid-call — and the control counts these collapses, because five flame losses inside a single heat call stop being individual events and become the one-hour lockout, 10 alternating with 13, in which gas heating is shut down for an hour. The same counting logic governs the family's edge with ignition: a rod too weak to prove flame at light-off does not produce a 13 at all but a failed ignition (11), and four failed ignitions reach the same 10 lockout by the other road — so a neglected 12 can present, weeks later, as either lockout pairing, and the stored history is what tells you which road the furnace took. The top rung, 14, does not climb from the others — it is a different failure altogether (flame or its false report with the valve off) — but it shares the lockout: once its condition clears, 10 + 14 follows for an hour. One reporting subtlety on communicating (EcoNet) systems belongs on this ladder because it hides its bottom rungs: Rheem does not display a flame loss (13) — or a failed ignition (11) — to the homeowner unless it has occurred at least three times within a single heat call, although the thermostat's active-fault screen shows it from the first occurrence and the control logs it once per heat call. So on an EcoNet system, 'the thermostat never showed anything' is compatible with a furnace quietly stacking 12s and early 13s at the board. Read the board display and the active-fault screen, quote the two-digit code and its count when you call, and the ladder becomes what it was designed to be: a warning system with time built into it, provided somebody reads the early rungs.

Shared causes: the rod, its wire, the ground — and the flame itself

Because the three codes watch one circuit, their causes form one list with two halves, and working it in Rheem's order prevents the classic misdiagnosis of this family — buying burner parts for an electrical proof failure. The proof half comes first. The rod itself: coated by seasons in the flame until its current fades (12), then fails intermittently (13); Rheem's remedy is cleaning or replacement, assigned to a qualified installer or service agency. Its connections and wiring: a rod not properly connected, wiring shorted or open — faults that imitate a dead rod exactly, and are found by inspection and continuity, not by replacing the rod and hoping. Its mounting: an improperly mounted sensor sits wrong in the flame and reports a healthy flame weakly. And the furnace ground: flame sensing depends on a sound ground reference, so a poorly grounded sensor or furnace corrupts the proof — which is why a flame code that arrives right after electrical work in the house deserves to have that work named when you call. The flame half comes second, and it is narrower than owners expect: an unstable flame pattern, proved by burner fit and the burner-door gasket and seals. A flame that lifts off the burners or flutters as the blower runs can leave the rod's envelope even with a perfect rod — but that conclusion is earned only after the proof half is eliminated, because the proof half causes most of this family's codes and all of its cheap ones. Note what is not on Rheem's lists for 12 and 13: the gas valve and the control appear only under 14, where the report itself (flame with the valve off) indicts them. Keeping the families separate is the diagnostic value of reading the exact number: a 13 sends the technician to a rod, a wire, a ground and a gasket; a 14 sends them to a valve, a miswire and a control. Arrive with the right number and the visit starts halfway down the right list.

What you can check, what Rheem assigns to a technician, and what to report

The owner's share of a flame code is observation, and on this family observation is unusually powerful — because the three codes are separated by timing, and timing is free to record. Establish which code you actually have, and read the whole display: a lone 12 between operating letters is the warning rung; a 13 is a mid-run loss; a 13 alternating with 10 is the lockout the losses earned; a 14 is a stop-everything event. Note when the loss happens if you witness a cycle: at light-off (which points back toward ignition and the weakest signals), seconds after the blower starts (the flame-stability branch), or deep into long runs. Note companions and history: 12s preceding 13s, electrical work preceding either, a recent service visit after which the codes began. On an EcoNet system, open the thermostat's active-fault screen — Rheem shows the fault there from its first occurrence even when the homeowner display is still silent. Then stop at the line Rheem itself draws. Rheem assigns flame-sensor cleaning to a qualified installer, service agency or the gas supplier; the rod, its wiring and the furnace ground are therefore technician checks, performed with the burner compartment open and a meter in hand, and everything under 14 — gas valve, 24VAC wiring at the valve, the control itself — is licensed gas-and-control work without an owner tier at all. What not to do generalises across the family: do not sand the rod on a forum's instructions and call the code fixed (a cleaned rod with a failing ground returns the same code), do not power-cycle a 13 repeatedly to keep heat limping through the night, and do not run — at all — a furnace that has shown 14. Report the code, its pairing, its count and its timing; on flame faults, that report is most of what the technician would otherwise have to wait and watch for.

When a flame code means stop: the family's red lines

Every family of codes has a point where 'book a visit' becomes 'shut it down', and Rheem's grading draws this family's lines explicitly. Code 14 is past the line by definition: extremely critical in Rheem's words, no operation, and a repair — wiring, valve or control — between the furnace and its next fire. A furnace in the 10 + 13 lockout repeatedly is at the line in practice: the control has counted five flame losses in one call and withdrawn gas heating for an hour, and a household that answers by cycling power after every lockout is re-offering gas to a burner whose flame proof has failed five times running. The same applies to a 13 pattern that shortens over days — losses arriving earlier in each cycle, then at light-off as 11s — because that slope is a proof circuit or a flame collapsing toward no-proof-at-all, and the lockout arithmetic is the only thing standing between that slope and continuous unproved gas flow. Carbon monoxide discipline overlays all of it: flame instability is a combustion-quality report as well as a sensing one, so soot at the burners, scorch marks, a lazy or lifting flame seen through the sight glass, or a CO alarm at any point moves any of these codes — even a humble 12 — into stop-and-call territory, with the gas utility added for a gas smell. The comfortable end of the family is real, and worth saying plainly: a lone 12 on a heating furnace is a scheduled repair, not an emergency; a first, single 13 is a prompt booking, not a midnight panic. The family's design gives you that calm precisely because its red lines are enforced by the control itself. Respect the grading Rheem printed — non-critical, counted, extremely critical — and you will neither ignore the warning rung nor fear it.

Key takeaways

• Read the grade with the number: 12 is non-critical and the furnace keeps heating, 13 is counted toward a lockout, and 14 is extremely critical with no operation — the same family, three different urgencies.
• On EcoNet systems, check the thermostat's active-fault screen: Rheem hides a first flame loss from the homeowner display until it has happened three times in one heat call, but the fault screen shows it from the first.
• Report the pairing, not just the code: 13 alone, 13 alternating with 10, and 12s in the history before the 13s are three different furnaces to the technician who takes your call.

Resetting the Rheem furnace

Rheem fault codes are shown as two-digit codes on the control's 7-segment display and clear once the fault condition is gone. Lockouts follow Rheem's documented rules: soft lockouts (the one-hour lockout, code 10, and the five-minute lockout when the low pressure switch fails to close after 60 seconds of inducer operation) auto-reset when their timer expires, and any lockout can be cleared immediately — once the original fault is cleared — by cycling power to the unit off and back on, or, for soft lockouts only, by cycling the heat call off for more than 2 seconds but less than 20 seconds. Hard lockouts are the exception: code 93 cannot be reset by cycling the heat call, and a dead-blower hard lockout (code 61, declared four times in one heat call) needs the repair plus a manual reset of power. Clearing a lockout without fixing its cause just restarts the count to the next one.

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Facts on this page were checked against manufacturer documentation: pts.myrheem.com · www.manualslib.com · pts.myrheem.com · pts.myrheem.com · www.manualslib.com · my.ruud.com:443