Navien NCB-40LDWE Common Problems & Fault Codes (2026 Guide)

The 6 most common faults

E352 High DIY-safe

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

What to try first

Locate a radiator in your home with a bleed valveOpen the valve using a radiator key to release a small amount of water into a clothCheck the boiler pressure gauge until it drops into the green zone (usually between 1.0 and 1.5 bar)Close the radiator bleed valve tightly

Est. cost: £80-150
Parts: Pressure relief valve, Pressure sensor (transducer), Filling loop

Full guide for E352

E351 High Engineer

Your boiler has been trying to refill its internal water supply for too long but has failed to reach the required level.

Est. cost: £150-250
Parts: Automatic filling loop valve, Pressure sensor, Expansion vessel

Full guide for E351

E762 High Engineer

Your boiler is struggling to detect the exhaust vent, likely because the flue pipe is not fitted securely or is the wrong size for your home.

Est. cost: £120-250
Parts: Flue assembly, Vent collar, PCB

Full guide for E762

E001 High Engineer

Your boiler has detected that the water temperature inside is getting dangerously high and has shut itself down to prevent damage or overheating.

Est. cost: £150-350
Parts: Primary heat exchanger, Flow sensor, Circulation pump

Full guide for E001

E003 High Engineer

Your boiler has failed to light the flame needed to heat your water and home.

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

Full guide for E003

E004 High Engineer

Your boiler has detected a flame when it shouldn't be burning, suggesting a sensor error or a gas valve issue that prevents the unit from starting safely.

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

Full guide for E004

Parts most often replaced

on the Navien NCB-40LDWE

Pressure relief valvePressure sensor (transducer)Filling loopAutomatic filling loop valvePressure sensorExpansion vessel

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

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See the full 52-code list for the Navien NCB-40LDWE