What should you check first?

If you smell gas, leave the area and contact your gas utility or emergency services from a safe location. Otherwise, note the full model number, the displayed code, whether other gas appliances work, and whether the heater recently lost power. Follow only the homeowner checks in your model’s manual. Do not open the sealed combustion compartment or adjust a gas valve.

A lack of hot water together with E003 warrants professional diagnosis. An ignition fault can involve gas supply, ignition components, wiring, combustion air, venting, or other model-specific conditions; the code alone does not identify which part failed.

Does Tustin hard water cause E003?

Hardness can contribute to scale in a heat exchanger over time, but E003 is an ignition fault. It is not proof that scale caused this shutdown. The City of Tustin 2026 water report (2025 testing) lists differing hardness measurements for its source waters; it does not establish one fixed grains-per-gallon value at every home. Check the water supplier and, if needed, test water at the property before selecting treatment or a service interval.

Read our Tustin water-hardness guide for what to measure and the repair page for the inspection process.

What should a repair visit establish?

Ask for a diagnosis tied to the exact Navien model and the fault history, an explanation of the test results, and an itemized repair recommendation. If a component is proposed, ask why the test points to it and whether warranty coverage applies. A flush is maintenance, not an automatic cure for an ignition fault.

For a no-hot-water situation, see the emergency repair options. For related codes, start with our multi-brand error-code guide.

Quick answers

Can I reset E003?

Follow the reset instructions for your specific model once you have checked for an unsafe condition. If E003 returns, stop repeated resets and arrange diagnosis.

Is E003 the same as E012?

No. Codes can distinguish failure to ignite from loss of flame after ignition; consult the exact model manual before interpreting either code.

Who can diagnose Navien E003 near Tustin?

A qualified tankless-water-heater service professional can check the model, fault history, and ignition system. Call +1 (855) 534-0035 to discuss the symptom and request an assessment.