A Navien NCB or NFC combi is a tankless water heater and a hydronic boiler sharing one burner, one fan and one control board, so its combustion codes are the same ones the tankless units throw. E003 ignition failure, E012 flame loss and E016 heat exchanger overheat are covered in depth in the Navien tankless code guide. This page spends its space on what that guide cannot cover: the closed heating loop, its pressure, its sensors and the valves that decide whether the burner's heat goes to the radiators or to the tap. The NHB and NFB heating-only boilers use the same heating-side numbers.
Read the loop pressure before anything else: on NCB-E combis and NHB boilers the display alternates between supply water temperature and loop pressure every five seconds, with a small psi marker lit when the number showing is pressure. The NCB-H shows system pressure on its front panel status screen. Navien rates these boilers for 12 to 30 psi at the boiler outlet, a typical house is filled to about 12 psi cold, and the heating side carries a 30 psi relief valve. A cold reading under 12 or a hot reading closing in on 30 is the real story, whichever code is on the screen.
Navien's published tables differ by series, and that catches people out: E302 (low water pressure) is printed in the NHB and NFB heating-boiler manuals. The NCB and NFC combi manuals we checked list E351, E352, E353 and E777 for pressure and water-level faults instead, because those units top themselves up through an auto feeder valve. Codes vary by model, and the label inside the front cover plus the manual for your exact model number win over this page. Parts and warranty notes sit on the Navien combi boiler repair page, every other make is listed on the brands hub, and you can book a visit here.
Navien combi boiler codes, decoded
| Code | What it means | Safe? | Try first | When to call |
|---|---|---|---|---|
E003 | Ignition failure. The burner tried to light and never proved flame. Shared with the Navien tankless line. | turn it off | Check that the gas valve under the boiler is open and that the stove or another gas appliance lights. Press reset once. | If it comes back after one reset. Do not keep cycling a burner that will not light. |
E012 | Flame loss. The burner lit and then dropped out mid-cycle. | turn it off | Look at the intake and exhaust terminations outside for snow, ice or a nest. | Same day in heating season. Gas supply under load, venting and the flame rod all need testing with instruments. |
E016 | Overheating of the heat exchanger. On the boiler side this is nearly always a water-flow problem, not a burner problem. | turn it off | Confirm the loop pressure is above 12 psi and that radiator or zone valves have not all been shut. | If pressure is normal and it repeats. A seized circulator, trapped air or a scaled exchanger needs a technician. |
E110 | Abnormal air pressure. The NHB manual words the same code as exhaust vent closed. | turn it off | Clear snow or ice from both vent pipes outside. Leave the pipes themselves alone. | If the terminations are clear. A blocked condensate path or a failing fan gives the same reading. |
E205 | Heating supply thermistor open or short circuit. The sensor on the pipe leaving the boiler for the radiators has stopped reporting. | turn it off | Nothing at home. The boiler cannot fire safely without this reading. | Book a repair. The sensor and its harness are tested with a meter and replaced if out of range. |
E218 | Heating return thermistor open or short circuit. The sensor on the pipe coming back from the radiators has failed or come unplugged. | turn it off | Nothing beyond one reset from the front panel. | If the code returns. The sensor, its connector and the harness back to the board are checked in turn. |
E278 / E279 | System supply (E278) or system return (E279) thermistor fault. These are the optional strap-on sensors on the house piping, not the ones inside the cabinet. | keep running | Look for a sensor wire that has been knocked off the pipe near the boiler. | At your next service if the house is heating normally; sooner if rooms are overshooting or lagging. |
E302 | Low water pressure in the heating loop. Printed in the NHB and NFB boiler manuals. | turn it off | Read the psi on the display and look under the boiler, at radiator valves and at the relief valve discharge pipe for drips. | If the pressure will not hold after a top-up. Water is leaving the loop or the expansion tank has lost its air charge. |
E351 | Abnormal auto feeder valve (make-up water). On NCB combis the boiler tried to refill the loop and could not finish in five minutes. | turn it off | Make sure the cold water supply valve to the boiler is fully open, then reset once. | If it returns. Either the feeder valve has failed or the loop is losing water faster than it can be replaced. |
E352 | High water pressure in the heating loop. | turn it off | Note the psi reading hot and cold. Do not cap or plug a dripping relief valve. | Promptly. A waterlogged expansion tank, a feeder valve passing water or a leaking domestic plate exchanger are the usual causes. |
E353 | Abnormal operation of the water pressure sensor. The boiler no longer trusts its own pressure reading. | turn it off | Nothing at home. | Book a repair. The sensor port can block with sludge on older, dirty loops, or the sensor itself has failed. |
E407 | Domestic hot water outlet thermistor open or short circuit. | turn it off | Try one reset from the front panel if the taps have gone cold. | Book a repair. Without this sensor the combi cannot control tap temperature. |
E434 | Abnormal operation of the water adjustment valve, listed in the NCB-H and NFC manuals. | keep running | Note whether hot water flow or temperature has changed, and tell the technician. | Within a few days. The motorised valve that meters domestic flow is sticking or has failed. |
E439 | Abnormal operation of the flow sensor. The part that tells the boiler a tap has opened is not reading properly. | turn it off | Clean the cold water inlet filter if your installer showed you how. | If hot water is erratic or absent. Scale and debris jam the sensor's turbine on hard water. |
E445 | Abnormal operation of the bypass mixing valve, listed in the NCB-H and NFC manuals. | turn it off | Nothing at home. | Book a repair. This valve blends the domestic outlet temperature, and Navien lists the fault as a manual reset. |
E740 | Abnormal operation of the outdoor temperature sensor used for outdoor reset. | keep running | Find the small sensor on an outside wall and check the wire has not been cut or pulled. | Non-urgent. The boiler heats without it but loses its weather-based supply temperature. |
E777 | Low water cutoff (LWCO). The boiler believes there is not enough water in it to fire. | turn it off | Check the pressure reading and confirm the make-up water supply is on. | Same day. Firing a boiler that is short of water destroys the heat exchanger. |
E782 / E784 | Communication fault with the main panel (E782) or with a Navien zone controller (E784). | keep running | Switch the boiler off for a minute and back on. | If the display or a zone stays dead. Wiring between boards, or the board itself, is at fault. |
The codes that usually mean a real repair
E302, E351 or E352: the loop will not hold its pressure
Pressure codes are the most common heating-side call on a Navien, and the boiler is rarely the part at fault. A loop that sags below 12 psi is losing water at a radiator valve, an air vent or the relief valve; a loop that climbs toward 30 psi hot has an expansion tank with no air cushion left, or a feeder valve letting city water creep in. The repair is the leak, the tank or the valve, and the price depends on which. There is no flat rate for hydronic parts: the diagnostic visit is $90–$150, folded into the repair if you proceed, and you get a written quote before any work starts.
E205, E218 or E016: sensors and circulation
A thermistor code means a sensor circuit is open or shorted, and replacing a clip-on sensor is one of the quicker boiler repairs. E016 beside either of them changes the picture: the exchanger is overheating because water is not moving, which points at the internal circulator, air trapped after a drain-down, or scale. Navien publishes no dedicated circulator code, so the pump is found by testing, not by reading the screen. On Halton and Hamilton hard water the domestic side also scales up, and it is cleared with the same descale flush a tankless gets, $150–$300.
No code at all: 3-way valve or flow sensor
Heat but no hot water, or hot water but cold radiators, with a clean display is the classic combi fault. A 3-way valve inside the cabinet sends boiler water either to the heating loop or to the domestic plate exchanger, and the flow sensor tells the board a tap has opened. When the valve motor sticks in one position, the boiler runs happily and heats the wrong circuit, so no code appears. Why a combi gives hot water but no heat walks through the checks, and boiler repair is the service that covers the valve, its actuator and the sensor.
Licensed technicians, a written price before the repair, no overtime charges.
Common questions
What should the pressure read on a Navien combi boiler?
Between 12 and 30 psi, and for most houses about 12 to 15 psi with the system cold and a few psi higher when it is hot. Navien's built-in low water cutoff needs at least 7.3 psi to let the burner run, and the heating-side relief valve opens at 30. A reading that swings more than about 10 psi between cold and hot usually means the expansion tank has failed.
Is E302 the same fault as E351?
They describe the same situation from two angles. E302 is the plain low-pressure code printed in Navien's NHB and NFB boiler manuals. NCB combis have an auto feeder valve that refills the loop on its own, so their manuals list E351 instead: the refill ran for five minutes and did not finish. Either way the question is where the water went.
Can I top up the pressure on a Navien combi myself?
If your installer showed you the fill valve and the display reads under 12 psi cold, bringing it back to 12 to 15 psi is reasonable, and it gets heat back for the night. Do it once. A loop is sealed, so pressure that drops again within days means a leak or a failed expansion tank, and repeated fresh water brings oxygen that corrodes the system.
How do I reset a Navien combi boiler after an error code?
Press the reset button on the front panel once the cause is dealt with; some codes clear on their own and others need that manual reset. If the same code is back after one reset, stop. Write down the number, the pressure reading and whether it happened on a heat call or a hot water call. Those three details shorten the diagnosis.
Is it safe to leave a Navien combi showing an error code overnight?
With E278, E279 or E740, yes: Navien treats those as alarms and the boiler keeps heating without the extra sensor. With a combustion, venting, pressure or low-water code, switch it off and leave it. If you smell gas or a carbon monoxide alarm sounds, leave the house and call Enbridge Gas or 911 from outside. Our emergency line is answered 24/7 with no overtime surcharge.