E04 = ignition trial failed after the configured number of attempts (typically 3-5). The control locks out for safety. Most common Lochinvar fault code.
Diagnostic sequence (in order)
1. Verify gas supply
- Gas shutoff at meter and at boiler — both fully open
- Other gas appliances (range, dryer) working — confirms gas to the house
- LP tank not empty (if propane)
2. Inspect igniter electrode
- Pull burner cover
- Visual: gap between electrode and ground = 0.10-0.12" typical (refer to manual)
- Eroded or cracked electrode = replace ($30-60 part)
- Carbon deposits on tip = clean with steel wool
3. Inspect flame sensor
- Flame sensor is the rod near the burner that detects flame presence
- Oxidation on sensor surface = clean with steel wool or fine sandpaper
- Bent or burned sensor = replace
4. Gas pressure verification
- Manometer test at gas valve inlet under fire
- 7" WC for natural gas; 11" WC for propane
- Low pressure = piping undersized, regulator failed, or supply issue
5. Gas valve
- Multimeter on gas valve coil during ignition trial — should energize
- No energization = control board issue
- Energized but no gas flow = gas valve mechanical failure (replace)
Common quick fixes
- Dirty flame sensor — most common; clean with fine sandpaper. 5-minute fix
- Eroded igniter electrode — replace at $30-60 part
- Closed gas shutoff — homeowners frequently forget
Reset
- Hold Reset button 5 seconds
- If E04 returns immediately = persistent fault; don\'t keep resetting
Bottom line
Most E04 cases: dirty flame sensor or eroded igniter. Annual service prevents both. Persistent E04 after these checks = gas pressure issue or gas valve.