How House Works

Noritz 903 (abnormal combustion): what to check

By How House Works EditorialBased on Noritz manualsChecked against sources Updated

Noritz display

903

“Abnormal combustion. The air inlet filter may be clogged.”

EZTR40 · EZTR40 Owner's Guide Rev. 01/17, p. 24; EZTR40 Technical Sheet EXSK072, p. 1
Owner checks, then technician

Quoted from the EZTR40 guide (EZTR40 Owner's Guide Rev. 01/17, p. 24); its technical sheet calls it “Combustion abnormality (Unit shuts off)” (EZTR40 Technical Sheet EXSK072, p. 1). The optional remote shows 90. Other models give 903 a different cause. NRC661A/NRC711 and NRC661: “Abnormal combustion. [When supplying combustion air from the indoors] The air supply vent may be clogged.” (NRC661A/NRC711 Owner's Guide Rev. 01/17, p. 25; NRC661 Owner's Guide Rev. 01/13, p. 25). NR501/NR662 and NER66OD: “Abnormal combustion.” (NR501/NR662 Owner's Guide Rev. 01/17, p. 24; NER66OD Owner's Guide Rev. 11/19, p. 24).

What it means

903 is an abnormal combustion code on Noritz heaters that show three-digit codes: the EZTR40, NRC661A/NRC711, NRC661, NR501/NR662 and NER66OD. Their guides put 901 and 903 in one row, with 908 too except in the NRC661 guide, and give 902 its own row for low gas supply pressure; the optional remote controller shows all of them as 90 EZTR40 Owner’s Guide Rev. 01/17, p. 24; NRC661 Owner’s Guide Rev. 01/13, p. 25. Noritz’s technical sheet says 903 shuts the unit off EZTR40 Technical Sheet EXSK072, p. 1.

The cause named in the row depends on the model. The EZTR40 guide says the air inlet filter may be clogged EZTR40 Owner’s Guide Rev. 01/17, p. 24. The NRC661A/NRC711 and NRC661 guides say the air supply vent may be clogged when combustion air comes from indoors NRC661A/NRC711 Owner’s Guide Rev. 01/17, p. 25; NRC661 Owner’s Guide Rev. 01/13, p. 25. The NR501/NR662 and NER66OD guides say only “Abnormal combustion.” NR501/NR662 Owner’s Guide Rev. 01/17, p. 24; NER66OD Owner’s Guide Rev. 11/19, p. 24.

Check these first

  1. EZTR40: turn the heater off with the power button or at the power supply and let it cool, then check the air inlet filter for dust, trash or similar build-up and, if it is clogged, clean it with a vacuum cleaner EZTR40 Owner’s Guide Rev. 01/17, p. 21. Do not remove the filter. If it has been removed or damaged, contact Noritz America; damage from running the heater without it is not covered by the warranty EZTR40 Owner’s Guide Rev. 01/17, p. 21.
  2. NRC661A/NRC711 and NRC661 that take combustion air from indoors: clean the air supply vent. The code table points to the cleaning instructions on p. 22 of each guide NRC661A/NRC711 Owner’s Guide Rev. 01/17, p. 25; NRC661 Owner’s Guide Rev. 01/13, p. 25.
  3. NR501/NR662 and NER66OD: the guides give no owner check for 903 NR501/NR662 Owner’s Guide Rev. 01/17, p. 24; NER66OD Owner’s Guide Rev. 11/19, p. 24.
  4. If the display continues, contact the nearest Noritz agent EZTR40 Owner’s Guide Rev. 01/17, p. 24. The EZTR40 guide also says to contact Noritz America if a code is indicated again after these actions; its code table gives 866-766-7489 EZTR40 Owner’s Guide Rev. 01/17, p. 24. The NER66OD code table gives 855-960-3434 NER66OD Owner’s Guide Rev. 11/19, p. 24.

The owner’s guides give no reset step for 903.

When to call a licensed technician

  • 903 comes back after the filter or vent is clean.
  • 902 appears. Its row says to have a professional check the gas supply pressure and to contact the nearest Noritz agent EZTR40 Owner’s Guide Rev. 01/17, p. 24. The EZTR40 installation manual gives 4–10.5 in. WC for natural gas and 8–14 in. WC for propane, and says the inlet pressure must be at least 5 in. WC (natural gas) or 8 in. WC (propane) for all appliances in the gas system EZTR40 Installation Manual Rev. 11/20, p. 16.
  • 908 appears. The technical sheets call it a combustion abnormality with the unit locked, reset by contacting the nearest Noritz agent EZTR40 Technical Sheet EXSK072, p. 1; NRC661A/NRC711 Technical Sheet EVPK072, p. 1.
  • Anything past the filter or vent. For 903 the sheet for service technicians lists checks for abnormal combustion and for blockage of the flue and air inlet, and gives its reset as disconnecting and reconnecting power EZTR40 Technical Sheet EXSK072, p. 1.

Notes for different models

  • On the technical sheets, 901 is a warning first and then a shutdown, while 903 and 902 shut the unit off EZTR40 Technical Sheet EXSK072, p. 1.
  • The NRC661 guide lists 901 and 903 only; its technical sheet also lists 908 NRC661 Owner’s Guide Rev. 01/13, p. 25; NRC661 Technical Sheet EVRK072, p. 1.
  • The EZ and NRCR documents do not list 903. Their guides print 901, 902 or 908 in the 90 row EZ/NRCR Owner’s Guide Rev. 10/25, p. 31. See the 90 page.

Related Noritz codes

  • 90 “(1) [When supplying combustion air from the indoors] The air supply vent may be clogged. (2) Exhaust vent may be clogged. (3) Abnormal combustion, low gas supply pressure. (4) Condensate drain line may be clogged.”
  • 902 “Abnormal combustion, low gas supply pressure.”
  • 908 “Abnormal combustion. The air inlet filter may be clogged.”

All Noritz codes

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Sources

  1. NORITZ America Corporation. EZ/NRCR Owner's Guide Rev. 10/25. SBB81M1, Rev. 10/25; cover title: Condensing Tankless Gas Water Heater Owner's Guide; covers EZ111DV, NRCR111DV, EZ98DV, NRCR92DV and EZ71DV (GQ-...-FF-1 US models); EZTR50 and EZTR75 are named only in the warranty table (p. 36); p. 1 is the unnumbered cover. Accessed Oct 12, 2026.
  2. NORITZ America Corporation. EZTR40 Owner's Guide Rev. 01/17. SBB809K-2, Rev. 01/17; covers NRC663-FSV (EZTR40). Accessed Oct 12, 2026.
  3. NORITZ America Corporation. NRC661A/NRC711 Owner's Guide Rev. 01/17. SBB8067-4, Rev. 01/17; covers NRC711-DV, NRC711-OD, NRC661A-DV and NRC661A-OD. Accessed Oct 12, 2026.
  4. NORITZ America Corporation. NRC661 Owner's Guide Rev. 01/13. SBB801H, Rev. 01/13; covers NRC661-DV and NRC661-OD. Accessed Oct 12, 2026.
  5. NORITZ America Corporation. NR501/NR662 Owner's Guide Rev. 01/17. SBB8065-5, Rev. 01/17; covers NR662-OD and NR501-OD. Accessed Oct 12, 2026.
  6. NORITZ America Corporation. NER66OD Owner's Guide Rev. 11/19. SBB818E-1, Rev. 11/19; covers NER66OD (GQ-2039WS ET US); contact lines name Thermalux Customer Care, 855-960-3434. Accessed Oct 12, 2026.
  7. NORITZ America Corporation. EZTR40 Technical Sheet EXSK072. Sheet code EXSK072, no revision or date printed; a service document filed under Field Manual in Noritz's Technical Literature; no page numbers printed, p. 1 is the PDF page. Accessed Oct 12, 2026.
  8. NORITZ America Corporation. NRC661A/NRC711 Technical Sheet EVPK072. Sheet code EVPK072, no revision or date printed; a service document; no page numbers printed, p. 1 is the PDF page. Accessed Oct 12, 2026.
  9. NORITZ America Corporation. NRC661 Technical Sheet EVRK072. Sheet code EVRK072, no revision or date printed; a service document; no page numbers printed, p. 1 is the PDF page. Accessed Oct 12, 2026.
  10. NORITZ America Corporation. EZTR40 Installation Manual Rev. 11/20. SBB809L-2, Rev. 11/20; covers NRC663-FSV (EZTR40), indoor installation; p. 22 is an image page without a text layer. Accessed Oct 12, 2026.