Rinnai Water Heater Error Codes

Common Symptoms

• Error code number displayed on the Rinnai front panel LED
• Unit powers on but will not fire — error code shown
• Unit fired briefly then shut down — error code appears after shutdown
• Intermittent fault — code clears on reset but returns
• Rinnai Control-R app shows fault history with timestamps
• Multiple codes appearing in sequence or combination

Most Likely Causes

1. 1

   Code 02 — No Ignition Detection (Flame Sensor Signal Not Received)

   Code 02 means the control board sent an ignition signal but received no confirmation from the flame sensor that a flame was established. Causes: (1) Dirty flame sensor rod — the most common cause; carbon and oxidation deposits prevent the ionization current from reaching the PCB. Clean the metal sensing tip with 400-grit emery cloth. (2) Loose or corroded flame sensor wiring harness connector. (3) Failed flame sensor rod — if cleaning does not resolve the code, replace the sensor. Code 02 is closely related to Code 11 — some Rinnai models use Code 02 for the initial no-detection event and Code 11 for the final three-attempt lockout.

2. 2

   Code 03 — Freeze Protection Activated

   Code 03 is informational — it means the Rinnai's built-in freeze protection circuit has activated because the ambient temperature near the unit dropped to the freeze threshold. The unit energizes small internal heaters to prevent the heat exchanger from freezing. Code 03 clears automatically when the unit warms up. Important: freeze protection only operates when the unit has AC power. If Code 03 appears after a power outage during freezing weather, perform a freeze damage inspection before restarting (close cold supply, slowly pressurize, check for heat exchanger leaks) — see the full procedure in the Rinnai water heater leaking guide.

3. 3

   Code 05 — Liquid Propane Solenoid Valve Fault

   Code 05 is specific to LP (propane) models and indicates a fault with the liquid propane solenoid valve — the valve that controls the secondary LP gas circuit. Causes: (1) Solenoid valve coil failure — test coil resistance with a multimeter (consult service manual for model-specific resistance spec; typically 20–50 Ω); replace the solenoid coil if reading is open or shorted. (2) LP gas supply pressure too low — Code 05 on LP models can appear if the propane tank is running low (below 20% capacity reduces regulator delivery pressure). (3) Wiring harness fault to the solenoid. Code 05 is a service-level code — on LP models, also verify the tank regulator output pressure meets the 8–14 in. w.c. specification.

4. 4

   Code 07 — Gas Valve Fault (Modulating Valve Control Circuit)

   Code 07 indicates a fault in the gas valve control circuit — either the modulating gas valve itself has failed or the PCB cannot communicate with or control the valve. The Rinnai modulating gas valve adjusts the gas flow continuously to maintain the setpoint temperature. Causes: (1) Failed gas valve solenoid — test resistance per service manual specs. (2) Wiring fault between PCB and gas valve — inspect harness for pinched or burned wires. (3) PCB output driver fault — if the valve tests good and wiring is intact, the PCB may have failed. Code 07 typically requires a service technician with Rinnai-specific test equipment for definitive diagnosis.

5. 5

   Code 10 — Air Pressure Switch Fault (Vent or Combustion Air Problem)

   Code 10 means the air pressure switch (also called the combustion air pressure switch or draft switch) did not confirm adequate airflow before or during the firing cycle. Causes in order: (1) Blocked vent termination cap — debris, ice, or a bird nest at the exterior vent cap is the most common cause; inspect and clear. (2) Blocked air intake — check the combustion air intake pipe for obstruction. (3) Vent pipe length exceeds maximum — if the vent run was recently modified, verify total equivalent length against the installation manual's published maximum for the model. (4) Failed air pressure switch — with the unit disassembled, the switch can be tested with a manometer; replace if it fails to actuate at the specified pressure. (5) Blocked condensate drain on RUR/RUS condensing models — a full condensate drain back-pressures the exhaust circuit, which can trigger Code 10.

6. 6

   Code 11 — No Ignition After Three Attempts (Gas, Igniter, Flame Sensor)

   Code 11 is the most common Rinnai fault code — the unit attempted ignition three times without establishing a stable flame and locked out. Diagnostic priority: (1) Gas supply — confirm the shutoff valve at the unit is fully open (handle parallel to pipe); check other gas appliances. Measure inlet pressure with a low-pressure manometer at the test port — natural gas must be 3.5–10.5 in. w.c. under firing load; LP must be 8–14 in. w.c. (2) Flame sensor rod — clean the metal tip with 400-grit emery cloth; carbon deposits cause false 'no flame' signals and are the single most common Code 11 cause. (3) Igniter electrode — inspect ceramic for cracks; verify spark gap (approximately 3–4 mm); replace if ceramic is damaged. (4) Flow rate below minimum — if no flow issue is obvious, install an inline flow meter to confirm the 0.4–0.5 GPM activation threshold is met. Reset with the on/off button (3-second press) or power cycle after each repair attempt.

7. 7

   Code 12 — Flame Loss During Operation (Gas Pressure Drop, Gas Valve)

   Code 12 means the burner ignited successfully but the flame extinguished during the heating cycle. Unlike Code 11 (failure to start), the unit runs briefly then drops out. Causes: (1) Gas pressure drop under full firing load — the burner consumes more gas at full BTU than at ignition; a marginal supply that maintains adequate static pressure may drop below the 3.5 in. w.c. minimum under load. Measure dynamic gas pressure while the unit is fully firing. (2) Partially failed gas valve — a sticking modulating valve can produce an inconsistent gas flow that sustains ignition but cannot maintain flame at demand. (3) Vent restriction — a partial blockage that allows ignition but starves the flame of combustion air at higher BTU demand. Inspect the exterior vent termination and the full vent run for sags or partial blockages.

8. 8

   Code 14 — Thermal Fuse Trip (Overheat — Descale Required on RUR/RUS)

   Code 14 means the thermal fuse (a one-time overcurrent safety device in the heat exchanger circuit) has opened due to an overheat condition. On non-condensing V and RU models: most common causes are a blocked flue gas restrictor, vent restriction, or recirculation of combustion gases into the combustion air intake. On condensing RUR and RUS models: the most frequent cause is calcium carbonate scale buildup in the secondary heat exchanger — scale acts as thermal insulation, creating hot spots that progressively lower the temperature at which Code 14 trips. Code 14 requires thermal fuse replacement (the fuse is a one-time device and does not reset). On condensing models, always perform the full heat exchanger descale (white vinegar or CLR, 60–90 minutes via service port isolation valves) BEFORE replacing the fuse — otherwise the new fuse will trip again.

9. 9

   Code 16 — Over-Temperature Fault (Outlet Temperature Exceeds 185°F)

   Code 16 means the heat exchanger outlet temperature exceeded 185°F — the Rinnai's over-temperature safety threshold. Unlike Code 14 (thermal fuse trip), Code 16 is a software-controlled shutdown that can reset. Causes: (1) Setpoint too high — verify the temperature setpoint is within the normal range (120°F for residential safety, up to 140°F with anti-scald precautions). (2) Low flow rate — if flow drops below the minimum during a draw, the water in the heat exchanger receives more heat per unit volume than intended, rapidly raising outlet temperature. (3) Thermistor fault — a failed outlet thermistor (Code 31) sending an incorrect reading can cause the PCB to underfire AND simultaneously trigger Code 16 if the actual outlet temperature is genuinely high. (4) Recirculation loop running at zero draw — if a recirculation pump is circulating water with no fixture open, the limited volume is heated repeatedly.

10. 10

    Code 25 — Cold Water Inlet Thermistor Fault

    Code 25 indicates the cold water inlet temperature sensor (thermistor) has failed or is reading outside its expected range. The inlet thermistor tells the control board the incoming water temperature so it can calculate the correct firing rate to reach the setpoint. A failed or out-of-range thermistor prevents accurate temperature control and triggers Code 25 shutdown. Check the wiring harness connector at the thermistor first — corrosion or a loose pin is a common cause of intermittent Code 25. If the connector is clean and secure, test the thermistor resistance with a multimeter and compare to the resistance-temperature table in the service manual (typically ~10 kΩ at room temperature). Replace with an OEM Rinnai thermistor if out of spec.

11. 11

    Code 29 — Condensate Neutralizer Fault (RUR/RUS Condensing Models)

    Code 29 is specific to Rinnai RUR and RUS condensing models and indicates a fault related to the condensate neutralizer — either the neutralizer housing is full, the media is exhausted and the condensate pH is too low, or there is a flow sensor fault in the condensate circuit. The condensate neutralizer is required by most plumbing codes to raise the pH of the acidic condensate before drain disposal. Fix: open the neutralizer housing and inspect the marble chip or calcium carbonate media — black, slimy, or fully consumed media must be replaced. Test condensate pH at the housing outlet with litmus paper; if pH is below 6.0 with fresh media installed, the condensate volume may exceed the neutralizer's capacity (common on combi-boiler models in heavy heating season).

12. 12

    Code 31 — Heat Exchanger Outlet Thermistor Fault

    Code 31 means the heat exchanger outlet thermistor has failed or is reading out of spec. This thermistor is the primary safety feedback sensor — the control board uses its reading to modulate the gas valve and prevent overheating. A Code 31 fault puts the unit into protective shutdown. As with Code 25, check the wiring harness connector first. Test thermistor resistance at room temperature (~10 kΩ typical) and compare to the service manual specification. On RUR/RUS models, Code 31 can appear as a secondary consequence of heavy scale buildup causing genuine outlet temperature anomalies — perform a descale first before replacing the thermistor to confirm the sensor is at fault.

13. 13

    Codes 32, 33, 34 — Combustion Air, Exhaust, and DHW Outlet Thermistors

    Code 32 is the combustion air inlet thermistor fault; Code 33 is the exhaust (flue gas) thermistor fault; Code 34 is the DHW (domestic hot water) outlet thermistor fault. These three sensor codes follow the same diagnostic approach: (1) Check the wiring harness connector for corrosion or looseness — re-seat firmly. (2) Test resistance with a multimeter against the service manual resistance-temperature table. (3) Replace the failed thermistor with an OEM Rinnai part. Code 33 (exhaust thermistor) is also worth investigating if there is a blocked or restricted flue — abnormally high exhaust temperatures from a partial blockage can cause the exhaust thermistor to fail prematurely.

14. 14

    Code 38 — Communication Error (PCB to PCB or to Peripheral)

    Code 38 indicates a communication error between the main PCB and a secondary component or module — most commonly the remote controller, the Control-R Wi-Fi module, or (on combi-boiler models) between the domestic hot water PCB and the heating PCB. Causes: (1) Loose or disconnected communication wiring between the two boards or between the PCB and the remote control. (2) Failed Control-R Wi-Fi module — try disconnecting the module and testing if Code 38 clears. (3) PCB failure on one of the communicating boards. Check all wiring harness connectors and re-seat any that show signs of looseness before ordering parts.

15. 15

    Code 41 — Bypass Servo Fault

    Code 41 indicates a fault with the bypass servo motor — the motorized valve that routes a portion of cold water around the heat exchanger to prevent scalding at high setpoints and to modulate outlet temperature precisely. If the servo fails to reach its commanded position, Code 41 is triggered. Causes: (1) Sticking servo motor from mineral scale on the valve body — this is more common on RUR/RUS condensing models; descaling the unit may free the servo if it is binding from scale. (2) Failed servo motor — test by checking the wiring harness and commanding the servo through a forced test if the service mode allows. (3) Wiring fault between PCB and servo. Code 41 is a service technician-level repair on most models.

16. 16

    Code 52 — Modulating Gas Valve Fault

    Code 52 indicates the modulating gas valve has failed or is receiving an incorrect signal from the PCB. The modulating valve is a proportional control valve that continuously adjusts gas flow rate from minimum to maximum BTU to maintain the setpoint temperature — unlike an on/off valve, it varies flow in response to water flow rate changes. Code 52 can result from: (1) Failed valve solenoid — test resistance per service manual spec. (2) PCB output driver failure — the PCB drives the modulating valve with a variable signal; a failed driver produces a constant or absent signal that the valve cannot follow. (3) Wiring fault between PCB and valve. Code 52 almost always requires a technician for definitive diagnosis and repair.

Quick DIY Checks

1. 1Step 1 — Read the code and access the Rinnai Control-R app fault history: note the code on the LED panel. If the Rinnai Control-R Wi-Fi module is installed, open the Control-R app (iOS/Android) → go to Device Settings → Fault History. The app shows every fault with timestamp and repeat count — far more valuable than the panel display alone. A code appearing at 6:00 a.m. every weekday suggests a gas pressure issue during morning demand peak. A code appearing only when ambient temperature drops below 40°F points to freeze protection or thermistor behavior at low temperatures. Screenshot the fault history before attempting any reset.
2. 2Step 2 — Perform the Rinnai reset procedure: press and hold the on/off button on the front panel for 3 seconds until the unit beeps and the display clears — this performs a soft reset. Alternatively, switch off the 120V circuit breaker to the unit, wait 30 seconds, and restore power (power cycle reset). After reset, attempt a hot water draw and observe whether the code returns immediately (persistent hardware fault), after a short delay (marginal condition like low gas pressure under load), or not at all (intermittent transient event). If the code returns after every reset, the root cause must be repaired before the unit will operate reliably.
3. 3Step 3 — Code 11/12 gas supply check: measure gas inlet pressure with a low-pressure manometer at the 1/8-inch NPT test port on the gas inlet valve. Measure static pressure (unit off, all appliances off) and dynamic pressure (unit firing at a full hot water draw — trigger by opening the largest hot tap in the home). Natural gas: static must be 3.5–10.5 in. w.c.; dynamic under full load must be ≥3.5 in. w.c. LP/propane: static must be 8–14 in. w.c. A static reading within range but a dynamic drop below the minimum means the gas supply is undersized for the peak load — contact the gas utility or a licensed plumber to address supply capacity.

1. 4Step 4 — Code 11 flame sensor cleaning: shut off the gas supply valve, close cold supply, unplug the unit. Remove the front cover. Locate the flame sensor rod in the burner assembly — a single metal rod electrode with one wire lead. Remove the single screw, slide the rod out. Clean the metal sensing tip only with 400-grit emery cloth or fine steel wool — remove all black or brown deposits. Do not sand the ceramic insulator body. Reinstall, restore gas and water, plug in, run a draw, and check if Code 11 has cleared. This cleaning step resolves Code 11 in the majority of cases.
2. 5Step 5 — Code 10 vent and air intake inspection: go to the exterior vent termination cap and inspect for debris, bird nests, insect nests, or ice. Clear any obstruction. Also inspect the combustion air intake cap if the model uses separate concentric or parallel vent pipes. Return indoors and check the vent pipe run for sagging sections that may trap condensate (on non-condensing models) or for a crushed, disconnected, or improperly routed section. If the vent run was recently modified, verify total equivalent length against the published maximum in the Rinnai installation manual for your model and BTU rating. On RUR/RUS condensing models, also inspect and clear the condensate drain line before further vent diagnosis — a blocked condensate drain can back-pressure the exhaust circuit and trigger Code 10.
3. 6Step 6 — Code 14 thermal fuse replacement and descale (RUR/RUS): if Code 14 appears on an RUR or RUS condensing model, perform the heat exchanger descale FIRST (white vinegar or CLR circulated for 60–90 minutes via the service port isolation valves — see Rinnai water heater not heating guide for full procedure). After flushing with clean water, attempt to reset the unit — Code 14 requires the thermal fuse to be replaced regardless of the root cause (the fuse is a one-time device). Locate the thermal fuse inside the heat exchanger assembly (exact location is model-specific — consult the Rinnai service manual); replace with an OEM Rinnai fuse only. After replacement and restoration, run a 10-minute hot water draw to confirm Code 14 does not recur.
4. 7Step 7 — Codes 25, 31, 32, 33, 34 thermistor diagnosis: shut off the unit and remove the front cover. Locate the thermistor identified by the code (consult the service manual or the wiring diagram on the unit door for sensor locations). Disconnect the wiring harness connector and re-seat firmly — a loose connector causes intermittent codes. Set a multimeter to resistance (Ω) mode. Measure across the thermistor terminals at room temperature (approximately 68–77°F / 20–25°C). Compare the reading to the resistance-temperature table in the Rinnai service manual for your model (typically ~10 kΩ at 25°C for most NTC thermistors). A reading of OL (open circuit) or near 0 Ω is a definitively failed thermistor. Replace with an OEM Rinnai thermistor for your model — aftermarket thermistors with slightly different resistance curves can cause offset temperature readings and recurring codes.
5. 8Step 8 — Code 29 condensate neutralizer maintenance (RUR/RUS): locate the condensate neutralizer housing (typically installed on the drain line below or beside the RUR/RUS unit, or on the wall nearby). Open the housing lid — it usually unscrews or unclips. Inspect the marble chip or calcium carbonate media: if it is black, slimy, dissolved to fine powder, or absent, it must be replaced. Install fresh neutralizer media (use media rated for condensate neutralizer service — not baking soda or other substitutes). Test the condensate pH at the housing outlet with litmus paper: value should be ≥6.0 with fresh media installed. Replace media annually as a preventive measure regardless of appearance.

Repair vs Replace

Recommended Tools & Parts

• Rinnai Flame Sensor / Igniter Assembly (Code 11/12, model-specific)

  OEM replacement spark igniter and flame sensor rod for Rinnai V, RU, RUR, or RUS series. Fixes persistent Code 11 or Code 02 after flame sensor cleaning. Verify exact model and serial number before ordering — part numbers differ by BTU rating and series.

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• Rinnai Thermal Fuse (Code 14, model-specific)

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• Rinnai NTC Thermistor (Code 25, 31, 32, 33, or 34, model-specific)

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• Condensate Neutralizer Replacement Media (Code 29)

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• Low-Pressure Manometer (Magnehelic or Digital, 0–35 in. w.c.)

  Required for measuring Rinnai gas inlet pressure at the 1/8-inch NPT test port — both static and under firing load. Essential for diagnosing Code 11 and Code 12 from gas pressure issues. Cannot be substituted with a standard gauge — residential gas pressure is too low for standard gauges to read accurately.

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Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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