Rinnai Control & Electronics Error Codes (FF, SE): Meaning & Fixes

Not every Rinnai display is a failure. This article covers the control-and-display family: FF, which is a service log entry a technician writes on purpose; SE, a cascade pointer that sends you to another unit's real code; SEt, a first-power setup demand on RX/CX models; 21 and 70, the genuine control faults around PC board replacement and failure; and 13, whose real identity corrects a common filing error. It closes with four displays — PC-03, P1, 01 and Null — that Rinnai's manuals do not document at all, and what to do if one seems to appear.

Displays that talk versus faults that stop

The control family divides into two species that must never be handled the same way. The first species is communication: the controller telling you something procedural. FF says a technician has been here. SE says the fault is on another unit in the cascade — look there. SEt says this RX/CX heater has never had its initial settings confirmed and will not proceed until an installer does it. None of the three stops a healthy heater, none is cleared by a reset ritual, and each has a correct, specific response that is not 'turn it off and on'. The second species is genuine control fault: 21, a data transfer that did not complete after a board change, and 70, the PC board's own failure verdict. Those two do stop the heater, and both live entirely inside the installer's world of data-transfer cables, parameter sheets and board replacement. The filing error this article also corrects — code 13 — belongs to neither species on a standard tankless heater: it is an RH180-hybrid combustion code that aggregator lists have scattered across the whole range. Holding the species apart is the entire skill this page teaches: respond to communication procedurally, respond to control faults professionally, and never let a listicle convince you a display means something Rinnai never printed.

Code FF — maintenance has been performed

FF is the only entry in Rinnai's code system that is written by a human on purpose. Rinnai's definition is exact: FF is a 'placeholder in diagnostic code history indicating that a service provider performed maintenance or service', and it is entered after servicing by pressing the UP, DOWN and ON/OFF buttons simultaneously. It appears when you scroll the last-9-codes history, sitting in sequence among any genuine faults, and its function is bookkeeping — it timestamps the point in the heater's story where a professional intervened. That makes FF quietly valuable to the next diagnosis. A history that reads 12, 12, FF, 12 tells you a service visit did not cure a flame failure, which redirects attention from 'has anyone looked at it' to 'what did the visit miss'. A history whose faults all sit before an FF, with clean running since, tells the opposite story. What FF is not: it is not a fault, it does not appear as a live shutdown display, it requires no reset, and a heater cannot 'have an FF problem'. If a service quotation proposes repairing an FF, the quotation is wrong. The one legitimate owner action concerning FF is to note where it sits in the sequence when transcribing the history for a technician — context that costs nothing and occasionally changes the diagnosis.

Code SE — cascade diagnostic display

SE exists only where several SENSEI commercial or SENSEI CX water heaters are cascaded — plumbed and wired to work as one plant, with one unit designated primary. Rinnai's definition: with cascade connections, the primary water heater's display flashes between 'SE' and the selected set temperature whenever an error code is displayed on any secondary water heater. SE is therefore a direction sign, not a diagnosis. The primary is healthy enough to be reporting; the fault lives on a secondary, which is displaying the actual two-digit code on its own controller. The correct response is procedural: walk the cascade, read each secondary's display, write down the real code and which unit shows it, and diagnose that unit with that code's procedure. Single residential heaters never show SE — there is no cascade and no secondary. Two practical consequences follow. First, 'the system shows SE' is never a sufficient service report; the useful report is 'unit 3 shows 12, primary shows SE'. Second, SE should not be reset at the primary: clearing the pointer while the secondary still faults just re-raises it. Cascade installations are commercial systems under service contracts in most cases, and SE is designed for that world — it routes the technician to the right cabinet in a plant room of identical cabinets.

Code SEt — initial settings not confirmed (RX/CX)

SEt is a commissioning display, not a malfunction. SENSEI RX/CX heaters ship configured by default for natural gas and indoor installation. The first time power is connected, the installer must confirm those settings or change them to match the actual installation — gas type, indoor versus outdoor — and until that is done the heater displays SEt. Rinnai frames the step with unusual force: if proper settings are not selected before the heater is used, 'it will cause a hazardous situation which results in personal injury or property damage', and only a trained and qualified professional should make the initial settings. The confirmation is done either from a smart device over Bluetooth with the Rinnai Central app — Rinnai's recommended route — or from the unit's controller, following the 'Initial Settings' section of the installation manual. For a homeowner, SEt has exactly two legitimate readings. On a brand-new or just-replaced heater, it means the installation is not finished: the installer comes back and completes commissioning. On a heater that has run for years, a fresh SEt after electrical work or a board change means configuration was lost or reset and must be re-confirmed by the same professional route — not guessed at. What SEt must never receive is experimentation: gas type is one of the settings behind it, and the whole point of the display is that a human qualified to choose it has not yet done so.

Code 21 — data transfer error after board replacement

Code 21 is a fault with a biography: it belongs to SENSEI and RX/CX models, and in practice it appears after a PC board has been replaced. A modern Rinnai board carries the heater's identity — its model parameters and, on RX/CX, its gas valve parameters — and a new board receives that identity by a data transfer process from the old board, using the transfer cable supplied with the replacement. Code 21 says that identity did not arrive intact: the transfer was not completed, the parameters were not properly programmed for the model in use, or the replacement instructions were not followed. Rinnai's remedy is correspondingly procedural rather than exploratory: ensure the data transfer process was completed; if it was unsuccessful, manually program all parameters per the PC board replacement instructions supplied with the board (on RX/CX, the gas valve parameter entry of the gas valve replacement instructions is called out specifically). Nothing in the 21 procedure involves the burner, the gas supply or the water — a heater showing 21 after a board swap has an unfinished installation, and the repair is to finish it with the paperwork that came in the box. Owners seeing 21 should read it as a callback, not a new breakdown: the correct call is to the installer who fitted the board, with the replacement instructions and parameter sheet, rather than to a diagnostic free-for-all.

Code 70 — PC board fault

Code 70 is the control family's true terminal fault, and across all three generations Rinnai's text for it is nearly identical in its brevity: check that all parameters were properly set; check every wiring harness at its connection to the PC board for looseness, damage or breakage; replace the PC board. The brevity is informative. By the time 70 is the honest finding, the board has failed its own supervision — this is not a code that points at a sensor or a valve, and the procedure does not send the technician on a component tour. What surrounds a 70 replacement is, however, substantial: the new board must receive the old board's data by transfer cable, or every parameter must be programmed manually from the replacement instructions (100000553 for SENSEI, 100000854 for RX/CX in the manual's references) — and a board fitted without that step is precisely how a 70 job turns into a 21 job. Owners should therefore hear a 70 quotation as a package: board, transfer, parameter programming, and verification firing. The two questions worth asking are whether parameters and harnesses were actually checked first (the procedure's steps 1 and 2, which occasionally rescue a board condemned by assumption), and whether the data transfer is included in the price. A 70 on a heater that has just had electrical work, a surge or water ingress also deserves its cause found — boards are sometimes victims, and a replacement fitted into a wet or shorting harness has a short life expectancy.

Code 13 — the member that is not a control code

Code 13 is included in this article to correct its filing, not to repeat it. In Rinnai's own Troubleshooting manual, code 13 is titled simply 'Combustion' and it applies to one product only: the RH180 hybrid tank-tankless water heater. Its procedure is a combustion procedure — 4-inch B-vent materials and installation, combustion air for the space, altitude DIP switches, inlet and forced low/high manifold pressures, fan inspection, burner orifice and chamber cleaning — with no control, display or board content at all. Standard tankless models (the V, RL, RU, RUR and SENSEI lines this site documents) do not use code 13. It migrated onto general 'Rinnai control error' lists through aggregator copying, and the migration matters practically: a homeowner with a standard tankless unit who is told they have a '13' almost certainly has a misread code (11, 12 and 14 are the visually confusable neighbours) or a photograph worth retaking. An RH180 owner with a genuine 13 has a combustion service call on a hybrid appliance, quoted against the RH180 manual. Either way, the professional response starts from the rating plate — model first, code second — which is the standing rule for every ambiguous display in this family.

Not Rinnai displays: PC-03, P1, 01 and 'Null'

Four displays assigned to this article could not be verified in any Rinnai tankless manual or tech sheet, and this site will not invent meanings for them. PC-03: no Rinnai document checked uses this form; Rinnai's system is two digits plus LC, SS, FF, SE and SEt, and PC-03's hyphenated shape belongs to other manufacturers' conventions. P1: no Rinnai documentation exists for it — P1 is documented in the tankless world as a Rheem low-water-flow display, and importing it onto a Rinnai would repeat the cross-brand error this site exists to correct. 01: Rinnai uses '01' prominently, but as data, not as a fault — item 01 of the performance-data mode is water flow rate in 0.1 gal/min units, and 01 is also a parameter number (maximum set temperature). A controller showing 01 while you hold buttons is showing you the maintenance monitor, not an error. 'Null': no Rinnai manual documents a Null fault display; the closest documented states are a blank display (a power or board matter, covered by the No Code procedures) and the '- - -' shown by an unassigned cascade secondary before its ID is set. If any of these four appears to be on your heater, the productive steps are the same: photograph the display exactly, note which buttons were being pressed, read the model from the rating plate, and put the photograph to a Rinnai-trained technician or Rinnai technical support. A display that cannot be matched to the manufacturer's own list is a question for the manufacturer — not an invitation to adopt another brand's answer.

Reading a control display like a technician

Technicians read this family in a fixed order, and it is worth borrowing. First, identify the species: is the display communication (FF, SE, SEt) or a control fault (21, 70)? Communication is answered with a procedure, never a part. Second, locate the speaker: on a cascade, SE means the speaking unit is healthy and the fault is elsewhere, so the reading is not finished until every secondary's own display has been checked. Third, ask what changed: 21 and SEt both have biographies — a board fitted, a commissioning left unfinished, power or configuration disturbed — and the change is usually the diagnosis. Fourth, only then consider the board itself, through 70's short procedure: parameters, harnesses, board. Owners who run those four steps before telephoning convert a vague 'the controller shows something odd' into a report a service desk can act on in one call: the exact display, the model, the unit's position in the cascade if there is one, and the last work done on the system. That is the entire craft of this family — none of its members rewards guessing, and all of them reward being read carefully. The same four steps protect you commercially: a control quotation that cannot say which species the display belongs to, or which unit in a cascade is actually speaking, has not yet earned its parts list.

Key takeaways

• FF in the fault history is a service timestamp, not a fault — note its position in the sequence when you transcribe the history for a technician
• On a cascade, never report just 'SE': walk the units, find the secondary showing the real two-digit code, and report unit number plus code
• A new heater displaying SEt has not finished being installed — commissioning (including gas type) is installer work with a hazard warning attached
• A code 70 quote should always include the data transfer and parameter programming; without them the same job returns as a code 21

Resetting the Rinnai water heater

The standard owner reset is a controller cycle: close the hot taps, press ON/OFF to turn the controller off, then on again, and run one test draw (code 03's documented form is ON/OFF twice). Special codes have special resets that are not interchangeable: LC and SS use five ON/OFF presses within five seconds (LC's version is a 70-hour temporary reprieve, not a cure); codes 17 and 52 clear only through the PC-board A/B-button sequence ending in 'ECL'; codes 57 and 58 need a hard reset — power off, then on — after the cause is corrected; and the LC hard reset after flushing is the A/B switches held together for five seconds. One reset, then observe: a code that returns is a live fault reporting itself, not a reset that 'didn't take'. Document the outcome of every reset you perform — cleared and held, returned during the draw, or never cleared — because that record is what turns a home reset into diagnostic evidence.

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Related guides

Facts on this page were checked against manufacturer documentation: www.rinnai.us · media.rinnai.us · www.rinnai.ca · media.rinnai.us · www.manualshelf.com · www.rinnai.us