Navien LCB700 Common Problems & Fault Codes (2026 Guide)

The 6 most common faults

E302 High DIY-safe

Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.

What to try first

Locate the external filling loop, which is a flexible silver hose connected to two valves under your boiler.Open both taps on the filling loop slowly to let water into the system while watching the pressure gauge.Close both taps firmly once the needle sits in the green zone, typically between 1.0 and 1.5 bar.

Est. cost: £80-150
Parts: Filling loop, Pressure relief valve, Expansion vessel

Full guide for E302

E352 High DIY-safe

Your boiler has detected that the water pressure inside the system is currently too high for it to operate safely.

What to try first

Locate a radiator in your home that is easy to access.Open the bleed valve on the radiator using a radiator key to release a small amount of water.Monitor the pressure gauge on the front of your boiler until it drops back into the green zone (typically 1.0 to 1.5 bar) and then close the valve.

Est. cost: £80-150
Parts: Pressure relief valve, Expansion vessel, Filling loop

Full guide for E352

E001 High Engineer

Your boiler has detected that it is getting too hot and has automatically shut down to prevent damage to its internal parts.

Est. cost: £120-250
Parts: Circulation pump, Heat exchanger, NTC thermistor sensor

Full guide for E001

E003 High Engineer

Your boiler is trying to start but the flame is failing to ignite, meaning you currently have no heating or hot water.

Est. cost: £120-250
Parts: Ignition electrode, Gas valve, PCB

Full guide for E003

E004 High Engineer

Your boiler has detected a flame inside when it is not supposed to be burning, so it has shut itself down as a safety precaution.

Est. cost: £150-300
Parts: Flame rectification electrode, Gas valve, PCB

Full guide for E004

E012 High Engineer

Your boiler has stopped because the internal flame has gone out, meaning it cannot heat your water or home.

Est. cost: £120-250
Parts: Ignition electrode, Flame rectification probe, Gas valve

Full guide for E012

Parts most often replaced

on the Navien LCB700

Filling loopPressure relief valveExpansion vesselCirculation pumpHeat exchangerNTC thermistor sensor

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

You can smell gas or see signs of a leak
The Navien shows an Emergency or High severity code
The boiler keeps locking out after repeated resets
You've tried the DIY checks and the fault hasn't cleared
There's visible water leaking from the boiler
The flame is yellow or orange instead of blue

See the full 32-code list for the Navien LCB700