Noritz Water Heater Error Codes

Common Symptoms

• Error code displayed on RC-7651M remote controller or unit LED panel
• No hot water with or without a visible error code
• Unit fires then shuts off immediately — Error 11 or 12
• Gradually declining hot water output — Error 90 scale buildup
• Error code visible only in Noritz Connect app fault history — intermittent fault
• Multiple error codes in sequence — diagnostic escalation from unresolved root cause

Most Likely Causes

1. 1

   Error 10 — Air Supply or Exhaust Blockage

   Error 10 indicates the Noritz detected a blockage in the combustion air supply or exhaust vent system. The unit monitors air pressure differential via an air pressure switch — if the switch does not close (confirming adequate airflow) within a set time during startup, Error 10 trips before ignition. Causes: blocked vent termination cap at the exterior wall (debris, ice, bird nest, spider web in the concentric vent rings); kinked, crushed, or improperly long vent run; failed draft inducer fan (check for Error 61 fan motor alongside Error 10); frozen condensate line on NRC/NRCP models blocking the vent; vent run with too many 90-degree elbows exceeding Noritz's maximum equivalent feet specification for your model and vent diameter. Fix: inspect the exterior vent termination first — clear any blockage from both the inner exhaust and outer air intake rings of the concentric terminal.

2. 2

   Error 11 — No Ignition (Gas Supply, Igniter, Flame Sensor)

   Error 11 means the Noritz attempted ignition three times without establishing a stable flame and locked out. Diagnostic hierarchy: (1) Gas supply — confirm shutoff valve is open (handle parallel to pipe); verify other gas appliances work; measure inlet pressure at the 1/8-inch NPT test port (NG: 3.5–10.5 in. w.c.; LP: 8–14 in. w.c.) both static and under firing load. (2) Igniter electrode — inspect ceramic insulator for cracks; verify spark gap ~3–4 mm; replace if damaged. (3) Flame sensor rod — clean the metal sensing tip with 0000 (four-aught) steel wool to remove carbon/oxidation deposits. This is the most common Error 11 cause after year one. To reset: power cycle the unit (press off, wait 30 seconds, press on). If Error 11 returns within a few firing cycles, the root cause has not been resolved.

3. 3

   Error 12 — Flame Loss During Operation

   Error 12 means the Noritz ignited successfully but the flame extinguished mid-cycle. Causes: (1) Dynamic gas pressure drop under full firing load — measure manifold pressure with the unit firing; NG must hold ≥3.5 in. w.c. under load. (2) Modulating gas valve fault (Error 71 may appear together) — valve holds flame at ignition but collapses at higher BTU demand. (3) Partial vent obstruction — blockage starves flame of combustion air at full output. Inspect both the exhaust and air intake vent terminals. (4) Flame sensor contamination — dirty flame sensor may signal flame loss when flame is actually present. Clean sensor with 0000 steel wool and retest.

4. 4

   Error 14 — Thermal Fuse (Overheat)

   Error 14 means the one-time thermal fuse inside the heat exchanger circuit opened due to an overheat event. On NRC/NRCP condensing models, this is almost always caused by scale-induced overheating (Error 90 precursor). The thermal fuse must be replaced — it does not reset. However, before replacing the fuse, complete the full heat exchanger descale procedure (see Error 90 section) or the new fuse will trip again. On NR non-condensing models, Error 14 more commonly indicates a blocked vent or flue restrictor. Always identify and resolve the root cause before replacing the fuse.

5. 5

   Error 16 — Overheating (Outlet Temperature Exceeds 203°F)

   Error 16 trips when the hot water outlet temperature exceeds 203°F (95°C) — a protective shutdown to prevent scalding and system damage. Causes: (1) Temperature setpoint too high combined with low flow rate — at very low flow rates, water spends more time in the heat exchanger and can overheat. Reduce setpoint to 120–130°F for household use. (2) Scale buildup in the heat exchanger reducing flow and creating hot spots — service with a descale. (3) Stuck-open gas valve delivering excess gas — Error 71 may also appear. (4) Failed outlet thermistor (Error 20 or 32) providing an incorrect low temperature reading that causes the control board to over-fire. Resolve the thermistor fault first before investigating other Error 16 causes.

6. 6

   Error 20 — Outlet Thermistor Fault

   Error 20 indicates the hot water outlet thermistor has failed or is reading out of range. This thermistor provides the control board with real-time outlet temperature feedback used to modulate gas valve flow. A failed outlet thermistor causes over-firing (too hot) or under-firing (too cold). Test: disconnect the wiring harness connector, inspect for corrosion or bent pins, and measure resistance across the thermistor terminals at room temperature — typically ~10 kΩ at 77°F (consult the Noritz service manual for your model's resistance-temperature curve). Open (OL) or near-zero reading = thermistor failed, replace with OEM part. A poor connector re-seat may clear intermittent Error 20 without a part replacement.

7. 7

   Error 21 — Inlet Thermistor Fault

   Error 21 indicates the cold water inlet thermistor has failed. This thermistor measures incoming water temperature, which the control board uses to calculate the required firing rate to reach the temperature setpoint. A failed inlet thermistor causes incorrect BTU delivery — water too hot or too cold — or a protective shutdown. Same diagnostic approach as Error 20: check connector for corrosion, measure resistance vs. temperature per the service manual. Replace with OEM Noritz inlet thermistor for your model — aftermarket thermistors with slightly different resistance curves cause persistent temperature offset.

8. 8

   Error 29 — Secondary Heat Exchanger Inlet Thermistor (NRC/NRCP Condensing)

   Error 29 is specific to NRC and NRCP condensing models and indicates a fault in the secondary (condensing) heat exchanger inlet thermistor. This thermistor monitors water temperature entering the secondary condensing heat exchanger. A failure can cause over-firing of the condensing stage or protective shutdown. Same diagnostic approach: check connector, measure resistance vs. temperature per the Noritz NRC/NRCP service manual. Also check for scale buildup in the secondary heat exchanger passages — heavy scale can cause genuine temperature anomalies that look like a thermistor fault. Perform a descale before replacing the thermistor on units with suspected hard-water scale.

9. 9

   Error 30 — Exhaust Thermistor Fault

   Error 30 indicates the exhaust gas thermistor (flue temperature sensor) has failed or is reading out of range. This thermistor monitors exhaust temperature to detect overfire conditions, vent blockage, and condensate issues on condensing models. A failed exhaust thermistor may cause the unit to run at abnormal temperatures undetected, or may trip Error 30 as a protective shutdown. Check the wiring harness connector first — exhaust thermistors are subject to heat cycling that degrades connector pins over time. Replace with OEM if the thermistor measures out of specification.

10. 10

    Error 31 — Combustion Air Thermistor Fault

    Error 31 indicates the combustion air intake thermistor has failed. This sensor measures the temperature of combustion air entering the unit and is used in the BTU modulation calculation. A failed air intake thermistor causes the control board to make incorrect firing rate decisions, potentially leading to over-heating or under-heating. Check connector first, then test resistance vs. temperature per the service manual. Also inspect the combustion air intake duct for blockage that could cause genuine air temperature anomalies.

11. 11

    Error 32 — DHW Outlet Thermistor Fault

    Error 32 indicates the domestic hot water (DHW) outlet thermistor specifically has failed. On Noritz models with multiple thermistors in the hot water circuit, Error 32 identifies the thermistor at the final DHW outlet point (downstream of the mixing valve on models with anti-scald mixing). Same diagnostic approach as Error 20. Replace with OEM part only — mixing valve temperature regulation is safety-critical and requires accurate thermistor readings.

12. 12

    Error 33 — Hot Water Temperature Sensor Fault

    Error 33 indicates a fault in the hot water temperature sensor — a secondary monitoring point distinct from the outlet thermistor. This sensor cross-checks the outlet thermistor reading for consistency. If the two sensors diverge beyond a tolerance threshold, Error 33 trips. Causes: sensor failure, wiring harness fault, or in some cases, a genuine flow anomaly causing the two sensors to read different temperatures. Check both sensors' wiring harnesses and test resistance on the Error 33 sensor.

13. 13

    Error 38 — Communication Error

    Error 38 indicates a communication fault between the main PCB and a secondary board — typically the remote controller (RC-7651M), the Wi-Fi module, or a secondary PCB inside the unit. Causes: (1) Loose or damaged communication wiring harness between the main board and the remote controller — inspect the two-wire communication line between the unit and the RC-7651M for damage, kinks, or loose terminals. Maximum remote controller cable run length is 160 feet for RC-7651M; exceeding this causes intermittent Error 38. (2) Failed RC-7651M remote controller — disconnect the remote controller temporarily and retest the unit without it; if Error 38 clears, the remote controller is the fault. (3) Failed secondary PCB inside the unit — requires professional diagnosis.

14. 14

    Error 59 — Solenoid Valve Fault

    Error 59 indicates a fault in one of the solenoid valves inside the Noritz — typically the water flow control solenoid or a gas solenoid sub-valve. Causes: solenoid coil failure (open or short circuit), solenoid plunger stuck due to scale or debris, or wiring harness fault. Test the solenoid coil with a multimeter — most Noritz solenoid coils read 10–20 Ω; an open reading (OL) indicates a failed coil. This error typically requires professional diagnosis and OEM part replacement — solenoid valve identification varies significantly by model.

15. 15

    Error 61 — Fan Motor Fault

    Error 61 indicates the combustion fan (draft inducer) motor is not spinning at the correct RPM or failed to start. The control board monitors fan speed via a tachometer feedback signal. Causes: fan motor bearing failure (unit will make a grinding or squealing noise before Error 61 trips), fan wheel blocked by debris or ice, wiring harness fault at the fan motor connector, or failed motor control circuitry on the PCB. Check the fan wheel for debris and ensure it spins freely by hand (with power off). If the fan is seized or makes noise, the motor assembly must be replaced. Noritz fan motor replacement requires full unit disassembly — typically a professional repair.

16. 16

    Error 65 — Flow Sensor Fault

    Error 65 indicates the water flow sensor is not reading correctly — either sending no signal or an erratic signal to the PCB. The flow sensor contains a paddle turbine that spins in the water stream; its spin rate signals flow volume to the control board. Causes: debris fouling the paddle turbine (most common — small particles lodge against the paddle), mineral scale on the paddle, or complete sensor failure. DIY fix: close isolation valves, depressurize, remove the flow sensor from the cold inlet manifold, clear debris from the paddle turbine, inspect the paddle for cracks or missing fins, reinstall, and retest. If the paddle is intact and clean but Error 65 persists, replace the flow sensor assembly.

17. 17

    Error 66 — Bypass Servo Fault (NRC/NRCP Models)

    Error 66 indicates a fault in the bypass servo motor — a motorized valve used on NRC and NRCP condensing models to modulate the proportion of hot water bypassing the condensing heat exchanger. The bypass servo optimizes condensing efficiency across different flow rates and inlet water temperatures. Causes: servo motor failure, servo gear strip, or wiring harness fault. This error typically requires professional diagnosis and OEM servo replacement — the bypass servo is model-specific and not a DIY-serviceable component.

18. 18

    Error 71 — Gas Valve Fault

    Error 71 indicates a fault in the modulating gas valve — either the valve is failing to respond to PCB commands (stuck open or stuck closed), or the valve's feedback circuit has failed. Error 71 may appear alongside Error 11 (no ignition) if the valve fails closed, or alongside Error 16 (overheating) if it fails open. Gas valve replacement is a licensed gas technician repair — do not attempt to service the gas valve yourself. Before condemning the gas valve, verify the wiring harness connector at the valve is fully seated and free of corrosion, as a poor connection is a common Error 71 cause.

19. 19

    Error 72 — Flame Sensor Fault

    Error 72 indicates the flame sensor circuit itself has failed — distinct from Error 11 (where the flame sensor works but detects no flame). Error 72 means the PCB cannot communicate with the flame sensor or the sensor circuit is open/shorted. Causes: sensor rod connection failure at the PCB harness, cracked ceramic insulator on the sensor rod causing a short to ground, or failed PCB flame detection circuit. Clean the sensor rod with 0000 steel wool and re-seat the wiring connector first — this resolves Error 72 approximately 30% of the time. If Error 72 persists after cleaning and connector check, replace the flame sensor rod assembly.

20. 20

    Error 90 — Combustion Abnormality (Scale — Full Descale Procedure)

    Error 90 is the most important Noritz error code on condensing NRC and NRCP models. It indicates combustion instability caused by calcium carbonate scale inside the condensing heat exchanger. Scale reduces heat transfer, creates localized hot spots, and destabilizes exhaust gas temperatures — all of which the Noritz control board detects as combustion abnormality. Error 90 progresses to Error 14 (thermal fuse) if untreated. Full descale procedure with CLR or white vinegar: (1) Turn off gas and electricity. (2) Close both cold inlet and hot outlet isolation service port valves (built-in on NRC/NRCP models). (3) Connect a submersible pump to the cold service port inlet; connect a return hose from the hot service port to a 5-gallon bucket. (4) Fill bucket with 3 gallons of undiluted white vinegar OR CLR Calcium, Lime & Rust Remover (dilute per label — typically 1:1 with water). (5) Open both service valves and circulate solution for 60–90 minutes. If the solution turns cloudy within 30 minutes, it has saturated — dump, refill with fresh solution, and continue. (6) Close service valves, drain the bucket, refill with 5 gallons of clean water, and flush for 5 minutes. (7) Restore operation. Noritz recommends annual descaling in areas with water hardness above 7 grains per gallon (120 ppm). In very hard water areas (above 15 GPG), descale every 6 months.

Quick DIY Checks

1. 1Step 1 — Read the error code and check Noritz Connect app history: note the error code displayed on the RC-7651M remote controller or unit LED panel. If the Noritz Connect Wi-Fi module is installed, open the Noritz Connect app (iOS/Android) and navigate to Fault History — the app shows each error code with timestamp and repeat count, going back 30 days. Codes that repeat daily at the same time indicate a recurring condition (e.g., Error 90 every morning during peak hot water demand = scale; Error 11 in cold weather = gas pressure drop at peak demand). A code that appears once and self-clears is less urgent than one that appears 10 times per week.
2. 2Step 2 — Reset procedure (power cycle 30 seconds): to reset a locked-out Noritz, press the power button to off, wait 30 seconds for the capacitors to discharge, then press on. If the unit fires and runs normally for several cycles, the error was a transient condition (brief gas pressure dip, brief flow rate drop). If the error returns within the first 1–3 firing cycles after reset, the root cause is still present and must be diagnosed. Never reset more than 3 times in succession without identifying the cause — repeated resets on Error 90 or Error 14 without addressing scale will eventually perforated the heat exchanger.
3. 3Step 3 — Error 10 (vent blockage): inspect the exterior vent termination — locate the concentric vent terminal on the outside wall. Clear both the inner exhaust ring and the outer combustion air intake ring of any debris, ice, bird nests, or spider webs. For high-wind areas, verify the terminal cap is not damaged or missing. Re-enter the unit area and inspect the full vent run for kinked or crushed sections. Count the number of 90-degree elbows and confirm the total equivalent feet does not exceed the Noritz specification for your model's vent diameter (consult installation manual). If Error 10 clears after the terminal is cleaned but returns seasonally, add a vent terminal guard.

1. 4Step 4 — Error 11/12 (ignition/flame loss): verify gas supply and measure inlet pressure (see Error 11 entry above for full procedure). Clean the flame sensor rod with 0000 steel wool. Inspect the igniter electrode. For persistent Error 12 with confirmed gas pressure, inspect the vent termination for partial blockage and test the bypass servo on condensing models (Error 66) — a failed bypass servo causes incorrect combustion air-to-gas ratio that can extinguish the flame.
2. 5Step 5 — Error 20/21/29/30/31/32/33 (thermistor codes): locate the specified thermistor on the unit (consult the Noritz service manual wiring diagram for your model — thermistor locations vary significantly between NRC, NRCP, and NR series). Disconnect the wiring harness connector, inspect for corrosion or bent pins, firmly re-seat, and retest. If the error persists, measure thermistor resistance with a multimeter at room temperature (typically ~10 kΩ at 77°F — consult the model-specific resistance-temperature table). Replace if open or shorted. Always use OEM Noritz thermistors — aftermarket parts with different resistance curves cause temperature offset and recurring faults.
3. 6Step 6 — Error 65 (flow sensor): close isolation valves, depressurize by opening a hot tap, remove the flow sensor from the cold inlet manifold (typically 1–2 screws plus harness connector), clear debris from the paddle turbine, and reinstall. If the paddle is cracked or missing fins, replace the sensor. After reinstalling, restore water supply and verify Error 65 clears on the next hot water draw.
4. 7Step 7 — Error 90 descale procedure (full CLR/white vinegar descale): follow the complete descale procedure described in the Error 90 cause entry above — close service valves, connect submersible pump, circulate 3 gallons of undiluted white vinegar or CLR for 60–90 minutes, flush with 5 gallons of clean water, restore operation. After descaling, power-cycle the unit and confirm Error 90 does not return on the first 5 firing cycles. If Error 90 returns within 24 hours of descaling, the heat exchanger may be perforated and require professional inspection. Schedule annual descaling to prevent recurrence.

Repair vs Replace

Recommended Tools & Parts

• Noritz Flame Sensor Rod

  OEM flame sensing electrode — clean with 0000 steel wool first; replace when cleaning no longer clears Error 11 or 72.

  $15–$40

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• Noritz Thermal Fuse Assembly

  OEM one-time thermal fuse for Error 14. Always complete Error 90 descale before installing — do not replace without resolving scale root cause.

  $25–$60

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• Noritz Thermistor (Inlet or Outlet)

  OEM thermistor for Error 20, 21, 29, 30, 31, 32, or 33. Specify your model and thermistor location when ordering — resistance curves vary by model.

  $20–$60

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• Noritz Flow Sensor

  OEM flow sensor paddle assembly for Error 65 or persistent no-activation at low flow.

  $30–$70

  Buy on Amazon →

• Descaling Kit (CLR + Pump + Hoses)

  CLR Calcium, Lime & Rust Remover plus a submersible pump and service hoses for Error 90 descale procedure.

  $30–$55

  Buy on Amazon →

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

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