Common problems guide

The most common Navien NHB 29 problems

The faults most likely to send a Navien NHB 29 into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

66 documented codes
6 most common
3 DIY-safe checks
3 engineer needed

The 6 most common faults

01
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
02
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
03
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
04
E783 High Engineer

Your boiler is having trouble communicating with your wall-mounted thermostat or smart control system, which prevents it from knowing when to turn on your heating.

Est. cost
£120-250
Parts
OpenTherm interface module, PCB, Communication wiring
Full guide for E783
05
001 High Engineer

Your boiler has detected an internal communication error and has safely shut itself down as a precaution.

Est. cost
£150-250
Parts
Main PCB, Communication wiring harness
Full guide for 001
06
003 High Engineer

Your boiler has failed to light the flame properly and has stopped attempting to heat your home or water for safety reasons.

Est. cost
£120-250
Parts
Ignition electrode, Gas valve, Flame sensor
Full guide for 003
Parts most often replaced

on the Navien NHB 29

Filling loopPressure relief valveExpansion vesselPressure TransducerFilling LoopExpansion 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
See the full 66-code list for the Navien NHB 29

Related guides

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