Navien Boiler Error Codes, Causes & How To Fix

Navien Boiler Error Codes: Full Guide to Causes and Fixes 2026

Quick Answer: The most common Navien boiler error codes are 02E and 302 (low water pressure), 10E (air pressure fault), and E787 (manual reset error). Low pressure codes can often be resolved by repressurising the system without an engineer. The 10E requires a Gas Safe engineer to investigate. E787 can frequently be cleared by repeating the reset process. All ignition, flame, heat exchanger, and component fault codes require a qualified engineer.

Navien is a South Korean manufacturer that has been growing steadily in the UK market and is gaining a strong reputation for build quality and efficiency in the combi and system boiler categories. Like all boilers, Navien units display fault codes on the control panel when something goes wrong, allowing homeowners and engineers to identify the source of the problem quickly. This guide covers all Navien error codes with clear explanations of what each means and what action to take.

Get a new boiler quote

Common Navien Boiler Error Codes

02E — Low System Pressure

The 02E code is one of the most frequently seen Navien fault codes and indicates that the system water pressure has fallen below the minimum threshold required for safe and effective boiler operation. Low pressure is one of the most straightforward boiler problems to address and is something most homeowners can resolve themselves without calling a Gas Safe engineer.

The first step is to check the boiler manual for the specific Navien NCB combi boiler model to confirm what the correct operating pressure should be — this can vary slightly between models. Once confirmed, locate the filling loop on the boiler and add water to the system slowly while watching the pressure gauge, closing the loop once the correct pressure level is reached.

If pressure drops again shortly after topping up, a slow leak somewhere in the central heating system is the most likely cause. In that case, a Gas Safe registered engineer should locate and repair the leak rather than the homeowner continuing to repressurise repeatedly as a temporary measure.

Can you fix it yourself? Yes — repressurise via the filling loop to the manufacturer’s recommended pressure. Call an engineer if pressure drops repeatedly.

10E — Air Pressure Fault

The 10E code indicates an air pressure fault within the boiler, most commonly caused by air pressure that is above the acceptable operating range. When this is detected, the boiler shuts itself down as a safety precaution to prevent further internal damage. There are three primary causes — a damaged air pressure hose, a clogged condensate water hose, or a faulty air pressure sensor. All three require a Gas Safe engineer to inspect, diagnose, and repair the affected component before the boiler is returned to service.

Can you fix it yourself? No — a Gas Safe engineer is required to diagnose and repair the fault.

302 — Low Water Pressure

The 302 code indicates low water pressure and is functionally equivalent to the 02E code, with the same cause and resolution. Adding water to the system via the filling loop to restore the pressure to the manufacturer’s recommended level is the first step and will resolve the fault in most cases without any engineer involvement. The same caveat applies — if pressure drops frequently after topping up, a leak investigation by a Gas Safe engineer is needed.

Can you fix it yourself? Yes — repressurise via the filling loop. Call an engineer if pressure drops repeatedly.

E787 — Manual Reset Error

The E787 code appears when a problem has been detected during the manual reset process. This can occur when the reset sequence is interrupted or not completed correctly, and in many cases simply repeating the reset process from the beginning will clear the fault and restore normal operation without any additional action.

If the E787 persists after a second complete reset attempt, or if it continues to reappear, the issue may be related to a deeper control system fault that requires a Gas Safe engineer to investigate.

Can you fix it yourself? Yes in many cases — repeat the reset process fully. Call an engineer if E787 returns after a second reset.

Full Navien Boiler Error Code Reference

Fault Code Detail: Key Secondary Codes

Code 3 — Ignition Failure

This code indicates the boiler has failed to ignite during the start-up sequence. Common causes include a faulty ignition electrode, a gas supply interruption, or a problem with the gas valve. Checking that other gas appliances in the property are working and attempting a single reset is a reasonable first step. If the fault persists, a Gas Safe engineer should inspect the ignition system.

Code 004 — False Flame Error

The 004 code means the boiler’s flame detection system is registering a flame signal when no flame should be present. This is a safety lockout that indicates a faulty flame detection electrode or a gas valve that is not fully closing when the boiler shuts down. Both require a Gas Safe engineer to diagnose and resolve, as a gas valve that is not closing correctly is a potential safety issue.

Code 012 — Flame Loss

The 012 code indicates the boiler’s flame has gone out during operation. This can be caused by a gas supply interruption, a faulty gas valve, a blocked flue, or a failing ignition electrode. Checking the gas supply and attempting a single reset is worth trying before calling an engineer. If the fault returns, a Gas Safe engineer should investigate the flame detection and gas delivery system.

Codes 016 and 046 — Overheated Heat Exchanger

Both codes indicate the heat exchanger has reached an unsafe temperature. This most commonly results from poor water circulation caused by a failing pump, a blocked or closed radiator valve, or very low system pressure. The boiler should not be restarted without a Gas Safe engineer confirming the cause of the overheat, as repeated overheating can cause permanent heat exchanger damage that is expensive to repair.

Code 030 — Abnormal Exhaust Temperature

This code indicates the flue exhaust temperature is outside the expected range. It can point to a heat exchanger fault, a flue obstruction, or abnormal combustion conditions. A Gas Safe engineer should inspect the flue and combustion system before the boiler is restarted.

Code 047 — Exhaust Thermostat Fault

The exhaust thermostat monitors flue gas temperatures as a safety measure. A fault with this component prevents the boiler from confirming safe flue gas expulsion and triggers a lockout. Thermostat replacement by a Gas Safe engineer is required.

Code 060 — Limit Switch Fault

The limit switch is a safety device that cuts power to the boiler if an unsafe condition is detected. A fault with this component requires inspection and replacement by a Gas Safe engineer before the boiler can safely be restarted.

Code 109 — Faulty Fan

The fan controls combustion air supply and flue gas exhaust. A fan fault prevents safe combustion and triggers a lockout. Fan motor failure, a wiring fault, or a faulty fan control board are the most common causes, all of which require a Gas Safe engineer.

Code 110 — Exhaust Blockage

An exhaust blockage prevents flue gases from being expelled from the property safely. While an engineer should be called, checking that the external flue terminal is not physically blocked by debris, bird nesting material, or ice is a practical first check the homeowner can carry out before the call-out, as a physically blocked flue terminal can sometimes be cleared without professional help.

Code 352 — High Water Pressure

High water pressure requires the system to be bled down to the correct level. Bleeding a radiator to release excess water from the system is the homeowner action, but if high pressure recurs without obvious cause — such as an overfilled system following recent repressurising — a faulty expansion vessel or pressure relief valve is the likely cause and a Gas Safe engineer should investigate.

Codes 205 and 218 — Thermistor Short Circuits

These codes indicate short circuits in the heating supply and return thermistors respectively. Short circuits typically indicate a wiring fault or a failed thermistor component, both of which require a Gas Safe engineer to diagnose and repair.

Codes 407, 421, and 480 — Hot Water and Cold Water Thermistor Faults

These codes indicate faults with the hot water outlet, cold water inlet, and hot water tank outlet thermistors respectively. All three monitor water temperatures at key points in the hot water circuit, and a fault with any of them will affect the boiler’s ability to deliver hot water at the correct temperature. Thermistor replacement is typically an inexpensive repair when the component itself has failed.

Codes 515 and 517 — Relay and Switch Faults

These codes indicate faults with the relay feedback circuit and dual in-line package switch settings respectively. Both are electronic control system faults that require a Gas Safe engineer with Navien product experience to diagnose and resolve correctly.

Code 594 — EEPROM Fault

The EEPROM is the memory chip that stores the boiler’s configuration and operating parameters. A fault with this component can affect all aspects of boiler operation and typically requires PCB-level repair or replacement by a qualified engineer.

Code 615 — Input and Memory Fault

Similar in nature to the 594, this code indicates a problem with the boiler’s input processing or memory systems. A Gas Safe engineer should diagnose the control system before any components are replaced.

Code 740 — Outdoor Temperature Sensor Fault

This code indicates a fault with the external temperature sensor used in weather-compensated heating systems. A failed sensor will prevent the boiler from adjusting output based on outdoor conditions, but the boiler may continue to operate in a default fixed-output mode depending on the control configuration. Sensor replacement is a straightforward repair.

Code 782 — Main Panel Communication Fault

This code indicates a communication failure between the boiler’s main control panel and another connected component or control module. A Gas Safe engineer with Navien system experience should diagnose the source of the communication breakdown before any parts are replaced.

FAQ

What does the Navien 02E fault code mean?

The Navien 02E fault code indicates that the system water pressure has dropped below the minimum required for safe boiler operation. It is one of the most common Navien fault codes and one of the few that homeowners can resolve themselves by repressurising the system via the filling loop to the pressure specified in the boiler manual. If pressure drops again shortly after being restored, a leak somewhere in the central heating system is the likely cause and a Gas Safe engineer should investigate.

Can I fix Navien boiler faults myself?

The 02E and 302 low pressure codes can both be resolved by the homeowner by repressurising the system via the filling loop. The E787 manual reset error can often be cleared by repeating the reset process. High pressure indicated by code 352 may be resolved by bleeding a radiator. All other Navien fault codes — including ignition failures, heat exchanger faults, fan faults, and thermistor faults — require a Gas Safe registered engineer to diagnose and repair.

What causes the Navien 10E fault code?

The Navien 10E fault code indicates an air pressure fault, most commonly triggered by air pressure within the boiler rising above the acceptable operating range. The three most likely causes are a damaged air pressure hose, a clogged condensate water hose, or a faulty air pressure sensor. All three require a Gas Safe engineer to inspect and repair. The boiler shuts down automatically when 10E appears to prevent further damage from sustained incorrect air pressure.

How do I repressurise a Navien boiler?

To repressurise a Navien boiler, locate the filling loop — the position varies between models and the manual confirms the exact location for each. Open the filling loop valve slowly while watching the pressure gauge and add water until the pressure reaches the level specified in the manual for the specific model. Close the filling loop valve firmly once the target pressure is reached, then attempt to restart the boiler. If the 02E or 302 code clears and the boiler operates normally, the fault was a straightforward pressure loss. If pressure drops again quickly, a leak in the system requires investigation.

What does Navien error code 012 mean?

Navien error code 012 means the boiler’s flame has gone out during operation and the boiler has entered a safety lockout. The most common causes are a gas supply interruption, a failing gas valve, a blocked flue, or a deteriorating ignition electrode. Checking that other gas appliances in the property are working normally and attempting a single reset is a reasonable first step. If code 012 returns after one reset, a Gas Safe registered engineer should inspect the gas delivery and flame detection system before the boiler is restarted.

How do I prevent Navien boiler fault codes?

Annual servicing by a Gas Safe registered engineer is the most effective way to prevent Navien fault codes from appearing. During a service the engineer will clean key components, check gas pressure and combustion quality, inspect the flue, test all safety devices, and identify any components approaching the end of their service life. Keeping the system pressure within the correct range and addressing any pressure drops promptly reduces the likelihood of 02E and 302 codes, and ensuring the condensate pipe is protected from freezing during cold weather prevents 10E faults triggered by blocked condensate hoses.

When should I replace my Navien boiler rather than repair it?

A Navien boiler under 10 years old with a single straightforward fault is almost always worth repairing. For boilers over 10 to 12 years old where multiple fault codes are appearing and major component replacements such as the heat exchanger or PCB are becoming necessary, replacement is likely the more economical long-term decision. A Gas Safe engineer can assess the specific boiler’s age, condition, and fault history to give an honest recommendation on whether repair or replacement represents the better investment.

Conclusion

Navien’s fault code system covers a comprehensive range of faults across their combi and system boiler ranges, and understanding what each code means is the fastest way to decide whether a quick DIY fix will restore operation or whether a Gas Safe engineer needs to be called. Of all the codes in this guide, 02E, 302, and E787 are the most likely to be resolved by the homeowner — by repressurising the system or repeating the reset process. All other fault codes require professional diagnosis and repair.

Annual servicing by a Gas Safe registered engineer remains the most reliable way to prevent Navien fault codes from appearing in the first place, keeping the boiler running efficiently, catching developing faults early, and extending the operational lifespan of the system throughout its expected 10 to 15-year service life.