Navien NCB500 Boiler: Faults, Error Codes and Breakdown Steps

The Navien NCB500 is a wall-mounted gas condensing boiler sold in combi and system versions, and most of what goes wrong with it shows up on the front panel as an error code. If the boiler has stopped, the first thing to try is a reset: press the Back button on the front panel¹. If the display shows a code, note it down before you do anything else, because it tells your installer or Navien's service desk what has happened.

If you can smell gas anywhere near the boiler, stop. Do not try to light any appliances, do not touch any electrical switch and do not use any phone in your building; call the National Gas Emergency Helpline on 0800 111999 from a neighbour's phone². The same number is the free 24-hour national gas emergency line used across the UK, whether the smell is inside or outside the property³. In Northern Ireland the number is different: call the NI Gas Emergency Service on 0800 002 001⁴.

For everything else, the boiler is designed to look after itself more than you might expect. After a power cut it restarts automatically and keeps your settings, with no manual reset needed¹. It has built-in frost protection, a monitored air-to-gas ratio to guard against carbon monoxide, and a 10 year warranty against manufacturing defects if it is registered within 30 days of installation¹.

If you smell gas near the boiler, call 0800 111999

A gas smell is the one fault on this page where you should not touch the boiler at all. Navien's own instruction is to leave the appliance alone and get out to a phone: do not try to light any appliances, do not touch any electrical switch, do not use any phone in your building, and immediately call the National Gas Emergency Helpline on 0800 111999 or your gas supplier from a neighbour's phone². The Health and Safety Executive gives the same number if you think an appliance is spilling carbon monoxide⁷.

Other official and network guidance adds practical steps. The Royal Voluntary Service advises opening doors and windows, turning off the gas at the meter unless the meter is in the cellar, not using electric switches, extinguishing naked flames and calling the National Gas Emergency Service immediately on 0800 111 999⁸. A local resilience booklet puts it more briefly: open doors and windows, leave the area and call the National Gas Emergency Line on 0800 111 999⁹. The National Gas Emergency Service should also be called if you experience low gas pressure or suspect carbon monoxide¹⁰.

The number is the same across Great Britain, and it is free and answered 24 hours a day³. Northern Ireland is the exception: Belfast City Council directs anyone who smells gas to the NI Gas Emergency Service on 0800 002 001⁴. For what happens after you ring, and how the engineer's visit works, see The National Gas Emergency Service: When to Ring and What Happens.

Resetting the boiler with the Back button

Most error codes on the NCB500 can be cleared, at least temporarily, with a reset. Navien's user manual says that if an error message appears you can try resetting the boiler to resolve the problem, and to reset it you press the Back button on the front panel¹. The system manual gives the same instruction⁶. The combi service manual describes the reset slightly differently, as pressing Reset on the front panel or turning the boiler's power supply off and then on again².

A reset is a first step, not a repair. If the same code comes back, the underlying fault has not gone away, and repeated resetting will not fix it. Note the code, check the simple things below, and call your installer if it returns.

Some codes are marked for manual reset and some clear themselves automatically. In the maker's tables, E003, E012, E016, E030, E031, E515, E615 and E788 are manual reset codes, while E004, E302 and E352 reset automatically¹. If you are unsure how many times it is sensible to keep resetting, read How Many Times Is It Safe to Reset a Boiler Lockout?.

Error codes: what each one means and how it resets

The NCB500 range uses a long list of codes, and the maker's manuals set out what each one means and whether it clears itself. The table below covers the codes a household is most likely to see. It is not a repair guide: anything involving the gas valve, the fan, the flue or the circuit board is a job for a Gas Safe registered engineer¹¹.

The combi service manual ties several codes to specific components: the ignition transformer is linked to E003 and E004, the flame rod assembly to E003, E004 and E012, and the main gas valve to E003 and E012². That is useful context for an engineer, not a reason for a householder to open the casing. For a wider explanation of how fault codes work across makes, see Boiler Fault Codes, Flashing Lights and Lockouts Explained.

Low or high water pressure: E302 and E352

Pressure faults are among the most common reasons an NCB500 stops, and they are also the ones a household can sometimes deal with. On the system version, if the pressure falls below the minimum set point, which is 1 bar by default, error code E302 appears on the display, and the manual says to fill with more water until the error message disappears¹². The combi service manual gives a target of 1.0 to 1.3 bar when repressurising².

The combi's low pressure setting range is 0.1 to 1.2 bar, with a default of 0.4 bar¹². High pressure shows as E352, which resets automatically¹. The water pressure sensor is linked to E351, E352 and E353².

Repressurising is done through the filling link, and the exact method depends on the filling loop fitted to your system. The general steps are covered in How to Repressurise a Combi Boiler and Boiler Pressure: What the Gauge Should Read and Fixing Low or High Pressure. If pressure keeps dropping after you top it up, there is a leak somewhere, and that needs an engineer.

Ignition failure and flame loss: E003 and E012

E003 means the boiler tried to ignite and failed; E012 means the flame was lit and then went out. Both are manual reset codes¹. On the combi, E012 is displayed if flame loss occurs 20 times consecutively, and it has to be cleared by hand². The older NCB service manual classes both as level 3 faults with manual reset¹³.

These codes usually point to something an engineer needs to check: the gas supply, the ignition parts, the flame sensor or the flue. A householder's useful contribution is to note when the code appears (on first lighting, after a power cut, in cold weather) and whether other gas appliances in the home are working. If they are not, the problem may be the supply rather than the boiler, and the guidance in What to Do When the Mains Supply to Your Home Goes Off applies.

Do not keep resetting an ignition fault indefinitely. If the boiler will not light after one or two attempts, book an engineer through Finding a Gas Safe Registered Engineer and Checking Their Card. In cold weather, a frozen condensate pipe can also stop the boiler, and that is covered in Frozen Condensate Pipes: Signs and How to Thaw One Safely.

After a power cut the boiler restarts on its own and keeps your settings

A power cut is not usually a boiler fault. Navien states that when the power is restored after a power failure, the boiler will automatically start and return to normal operation, and a manual reset is not required¹. The same wording appears in the system manual⁶. The boiler will also retain your settings during a power outage, including temperature settings¹.

That means you should not need to reprogramme anything after the lights come back on. If the boiler does not restart, check that the power is actually back at the boiler's fused spur, and check the fuse: the maker specifies a 3 A fuse on the supply side and a 3.15 A fuse in the boiler fuse holder¹². The boiler runs on a 230V/50Hz supply and draws a maximum of 115 W across all four system models⁵.

If your heating controls or programmer have lost their settings after an outage, that is a separate issue from the boiler itself; see Why Your Programmer Reset After a Power Cut. For a wider look at managing without power, the Power Cuts: A Household Guide section covers lighting, heating and keeping warm.

Built-in protection against frost, overheating and carbon monoxide

The NCB500 has several protections built in, and knowing what they do helps you judge when to call someone.

• Frost protection. A sensor inside the boiler automatically detects the temperature and, if necessary, initiates a safety heating cycle to prevent internal equipment damage due to freezing temperatures¹. The combi's freeze protection setting range is 6°C to 10°C².
• Overheating protection. The boiler monitors its own temperatures and raises codes such as E001, E016 and E031 if things get too hot¹. On the combi, E001 is displayed if the space heating water temperature is higher than 105°C².
• Carbon monoxide protection. The boiler is designed to maintain a safe air-to-gas ratio and combustion rate, continuously monitored by its air ratio control module¹.
• Electrical protection. The boiler has an electrical protection degree of IPX4D, and each boiler is specially grounded, both internally and externally, to protect against lightning strikes¹².

The boiler is approved for use in the UK and Ireland only, and using it elsewhere voids the warranty¹¹. It must not have petrol or other flammable vapours and liquids stored or used in its vicinity¹¹. For the wider picture on carbon monoxide and gas safety at home, see Home Emergencies: Fire, Carbon Monoxide, Gas Leaks and Burst Pipes.

Hot water temperature, hard water and everyday care

The combi's hot water temperature is set to 50°C at the factory for safety and comfort¹². Navien warns that water temperatures at or above 52°C can cause instant scalding or severe burns, and gives a table of burn times for a young child: less than 1 second at 70°C, 1 second at 60°C, 10 seconds at 55°C and 10 minutes at 49°C². The maker's advice is to use the lowest operating temperature setting necessary to provide comfortably hot water, to consider a lower setting where there are children or elderly or disabled residents, and to feel the water before using it on them².

In hard water areas, where hardness exceeds 200 ppm, Navien says it is important to install a scale reducer, and the system manual adds that you should seek the advice of the local water authority². The combi manual gives the boiler's water quality range as 1 to 12 °dH, which is 17 to 224 ppm².

For cleaning, the instruction is the same across the manuals: clean the outside of the boiler with a damp cloth with a little detergent, and do not use abrasive products¹. Nothing inside the casing is a household job.

Combi and system versions, servicing and the 10 year warranty

The NCB500 comes as a combi and as a system boiler, and the difference is set on the circuit board: SYSTEM is ON and COMBI is OFF on DIP switch 1². Both are wall-mounted and run on natural gas or LPG, with an LPG conversion kit included¹¹. If you are choosing between the two types, Combi or System Boiler: Which Keeps Hot Water Going in an Outage sets out the trade-offs.

Servicing and repair must be done by a Gas Safe registered installer, a service agency or the gas supplier¹¹. If the boiler develops a fault, Navien says your first action should be to contact your installer, as their professional assessment matters; if you cannot, phone Navien Service on 0344 332 2323 or visit www.navien.co.uk². For warranty service, contact the Technical Helpdesk first, with the model number, serial number, date purchased, installation location and type, and any error code to hand².

The warranty runs for 10 years from the installation date for manufacturing defects, for residential use in a single dwelling⁵. To register, use the Navien Installers Warranty App within 30 days of installation, or email a photo of your completed benchmark form to service@navienuk.com¹. The boiler also has a service notification setting, adjustable from 30 to 3650 days, with a default of 1820 days (5 years)¹².

When an old boiler is replaced, Navien states that neither the wall-mounted boiler nor its accessories are to be disposed of with domestic waste; hand it in at a designated collection point such as a duly authorised reseller or an authorised collection site for recycling, and observe the applicable national regulations⁶.