Common problems guide

The most common Navien NCB 40 HE problems

The faults most likely to send a Navien NCB 40 HE into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

108 documented codes
6 most common
4 DIY-safe checks
2 engineer needed

The 6 most common faults

01
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
02
E02 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 filling loop (usually two silver flexible hoses) underneath the boiler.Open the valves slowly to allow mains cold water into the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar).Close both valves tightly to stop the flow of water.

Est. cost
£80-150
Parts
Filling loop, Pressure relief valve, Expansion vessel
Full guide for E02
03
E302 High DIY-safe

Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

What to try first

Locate the internal filling loop (usually a silver braided hose or a plastic linkage under the boiler).Slowly open the valves to allow mains water into the system until the pressure gauge reads between 1.0 and 1.5 bar.Close the valves firmly and ensure no water is leaking from the connection points.

Est. cost
£80-150
Parts
Filling loop, Pressure relief valve, Expansion vessel
Full guide for E302
04
E002 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, usually found underneath the boiler.Gradually open the filling loop valves to allow water into the system while watching the pressure gauge.Stop filling once the needle reaches the green zone, typically between 1.0 and 1.5 bar.Ensure both valves are fully closed and tight.

Est. cost
£80-120
Parts
Pressure Transducer, Filling Loop, Expansion Vessel
Full guide for E002
05
E01 High Engineer

Your boiler has failed to light the flame properly, meaning it cannot produce heat or hot water.

Est. cost
£120-250
Parts
Ignition electrode, Gas valve, Flame sensor
Full guide for E01
06
E121 High Engineer

Your boiler has stopped because its internal sensors have detected that the water inside the system is not circulating correctly.

Est. cost
£150-250
Parts
Central heating pump, Flow sensor, Diverter valve
Full guide for E121
Parts most often replaced

on the Navien NCB 40 HE

Pressure relief valvePressure sensor (transducer)Filling loopExpansion vesselPressure TransducerFilling Loop

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 108-code list for the Navien NCB 40 HE

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →