E02 = system water pressure below threshold (typically < 10 PSI). The combi boiler locks out to protect against dry-firing.
Diagnostic sequence
- Read the pressure gauge. Should be 12-15 PSI cold; if below 10, E02 trips
- Open the manual fill valve until gauge reads 15 PSI. Close valve
- Bleed air at radiator/baseboard high points using bleeder keys. Continue until water flows steady (no hissing)
- Re-pressurize if bleeding dropped pressure below 12 PSI
- Reset E02 on the boiler control panel
- Monitor pressure for 24 hours. If pressure drops again, system has a leak
Finding leaks
- Visual at radiators, valves, fittings — wet spots, calcium deposits
- Expansion tank — failed bladder = waterlogged tank = pressure-relief discharge. Check pre-charge with tire gauge (12-15 PSI typical)
- Pressure-relief valve discharge — wet tube = relieving
- Below-floor radiant tubing leak — harder to find; pressure-test and visual inspection
Auto-fill valve
Most Lochinvar installs include an auto-fill valve set to maintain 12-15 PSI. If E02 trips, the auto-fill may be: closed (turn handle to open), failed (stuck), or downstream isolation valve closed. Verify auto-fill operation.
Bottom line
E02 = re-pressurize + bleed + look for leaks. If pressure won\'t hold, get a contractor for system leak detection.