Code 003 (or E003) = No flame / ignition failure. The unit attempted to fire and didn't detect flame. The highest-volume Navien error code by support-call frequency.
Diagnostic sequence
1. Gas supply
- Confirm the gas valve at the unit is fully open
- Test another gas appliance — if all are out, utility supply issue
- Check propane tank level if LP — low LP throws 003 reliably
2. Air in the gas line
Recently turned off (new install, repair, utility outage)? Purge by running another gas appliance for 5-10 minutes, then attempt Navien restart.
3. Gas pressure
Navien needs minimum 4" WC inlet on NG, 10" WC on propane, sustained during fire. Undersized gas line is the #1 chronic cause. Licensed plumber tests with a manometer.
4. Flame rod (#1 hardware cause on older units)
The flame rod detects flame via micro-current. Carbon buildup drops the signal below detection threshold. Cleaning the rod with fine sandpaper resolves about 40% of chronic 003 cases. See flame rod page.
5. Igniter
Direct spark igniter wears out. Check for spark when the unit attempts to fire (dark room, sight glass). No spark = igniter or igniter cable. See igniter page.
6. Gas valve
Less common but happens. Stuck-closed solenoid; replacement is dealer-only.
Reset procedure
- Address the suspected cause
- Press Reset on controller (Level 1 reset)
- If code returns immediately, escalate to next cause
- Monitor for 24 hours under normal use
Common scenarios
- Code 003 only at high flow: gas line undersized — code triggers when modulation demands full burn
- Code 003 after cold snap: propane tank pressure dropped — tank too small or regulator iced
- Code 003 after install: air in line, undersized line, or no gas pressure test
- Code 003 in old unit (10+ years): flame rod or igniter end-of-life — routine replacements
Combi boiler specific
On NCB / NFC combi units, Code 003 affects both heating and DHW (burner is shared). If only one mode fails, the issue is more likely the 3-way diverter or DHW flow sensor, not Code 003.
Bottom line
Work the diagnostic sequence top-down. Flame rod cleaning is free and resolves a huge fraction of cases. Persistent code requires Navien support or an NSS.