Navien E109 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E109 fault code mean?
The E109 code on a Navien boiler indicates that the fan motor is not reaching the correct operating speed. Your boiler's fan is responsible for drawing in fresh air for combustion and expelling harmful flue gases safely outside your home. If the control board detects that the fan isn't spinning at the required speed, it triggers a safety lockout and prevents the boiler from firing. This protects your household from the risks of incomplete combustion and carbon monoxide build-up.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Dirty or worn fan assembly Common
Dust and debris accumulating on the fan blades or motor bearings over time can slow the fan or cause it to draw excessive current. This is the single most common trigger for E109 on Navien boilers, particularly on units that haven't been serviced recently.
- Blocked air intake or flue vents Common
Any obstruction at the external vent terminals — such as compacted debris, bird nesting material, or ice in cold weather — restricts the airflow the fan is trying to move, preventing it from reaching the target speed. A visual check outside is the first practical step.
- Faulty or clogged air pressure switch (APS) Sometimes
The air pressure switch monitors airflow by detecting the pressure differential created by the fan. If the APS diaphragm is faulty, or its port is clogged with condensation residue, it may signal incorrect fan operation even when the fan itself is fine.
- Damaged or disconnected APS tubing Sometimes
The small rubber or plastic tube connecting the air pressure switch to the fan housing can crack, kink, or work loose. If the tube can't hold pressure, the APS receives a false signal and the boiler shuts down for safety.
- Wiring fault between fan, APS, and PCB Sometimes
Corroded terminals, loose connectors, or chafed wiring in the fan circuit can interrupt the speed-feedback signal to the PCB. The board sees incorrect speed data and triggers E109 as a precaution.
- Failed fan motor Rare
If the motor windings are damaged or the motor has simply reached end of life, the fan will not spin at all or will stall shortly after starting. This requires a full fan unit replacement by a Gas Safe engineer.
How to fix it
- Check your gas supply is on DIY safe
Confirm the gas isolation valve at the meter is fully open and, if possible, that other gas appliances in your home (such as a hob or gas fire) are working normally. If there is no gas supply to the property, contact your gas supplier rather than an engineer.
- Inspect the external vent terminals for blockages DIY safe
Go outside and look at where the boiler's flue pipes exit the building. Clear away any obvious debris, leaves, or ice from around the intake and exhaust terminals. Do not insert tools or fingers into the pipes. If you spot a bird's nest or ice blocking the flue, remove the accessible material carefully and allow the boiler to cool before resetting.
- Reset the boiler once or twice DIY safe
On most Navien models, press the Back button on the front panel to reset. Listen carefully after the reset — you should hear the fan spin up before ignition is attempted. If the boiler fires successfully, monitor it over the next few hours. If E109 returns, do not keep resetting repeatedly, as this can stress components and may affect your warranty.
- Check and clean the air intake filter (if accessible to a competent homeowner) DIY safe
Some Navien models have a removable air intake filter on the front or side of the casing. If your manual shows this as a user-serviceable item, remove it carefully, rinse it under clean water, allow it to dry fully, and refit it. Do not attempt to open the main casing or touch any internal components.
- Engineer to inspect the fan motor and measure PCB output voltage Gas Safe engineer
A Gas Safe engineer will check that the PCB is delivering the correct voltage to the fan motor. If the voltage is within specification but the fan still doesn't reach the required speed, the fan unit itself is confirmed as the faulty component and will need replacing.
- Engineer to inspect and test the air pressure switch and tubing Gas Safe engineer
The engineer will check the APS tubing for cracks or disconnections, test the switch's pressure response, and replace the APS or tubing if either is found to be faulty.
- Engineer to check wiring connections throughout the fan circuit Gas Safe engineer
All connectors between the fan, APS, and PCB will be inspected for corrosion, loose pins, or heat damage. Faulty wiring looms or connectors will be repaired or replaced as necessary.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If the homeowner steps above haven't resolved E109, or if the code keeps returning after resets, this fault requires professional diagnosis. Do not attempt any further resets or internal inspection yourself. A Gas Safe engineer can fully test the fan, APS, PCB outputs, and flue integrity and carry out any necessary replacement work safely and legally.
Parts you may need
- Navien fan motor assembly · from £145
- Air pressure switch (APS) · from £45
- APS connecting tubing/hose · from £12
- Fan wiring loom/connector · from £25
- Air intake filter · from £15
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £180–£375, depending on the underlying cause.
Frequently asked questions
Can I fix the Navien E109 error myself?
The safe homeowner steps are limited to checking your gas supply, clearing any obvious blockage at the external vent terminals, cleaning the air intake filter if your model has a user-accessible one, and resetting the boiler once or twice. Everything beyond that — inspecting the fan motor, testing the air pressure switch, checking wiring or measuring PCB voltages — must be carried out by a Gas Safe registered engineer, as it involves internal gas-side components.
How much does it cost to fix a Navien E109 fault in the UK?
Most people with an E109 fault end up paying somewhere between £180 and £375 all in, depending on what needs replacing. A straightforward air pressure switch swap sits toward the lower end, while a full fan motor replacement including parts and labour typically falls in the £225–£375 range. If the PCB is also found to be faulty, costs can rise considerably higher — worth bearing in mind on older boilers where it may be more economical to replace the boiler entirely.
Why does my Navien keep showing E109 after I reset it?
A fault that clears on reset but quickly returns almost always means the underlying cause hasn't been addressed. Repeated E109 codes usually point to a fan motor that is wearing out, a persistently blocked vent, or a failing air pressure switch. Repeatedly resetting without a proper repair can stress other components and may invalidate your warranty. If it returns more than twice, call a Gas Safe engineer rather than continuing to reset.
Is the Navien E109 code dangerous?
The boiler has gone into a safety lockout, which means it has deliberately shut itself down rather than attempt combustion without adequate airflow. In that sense the lockout itself is a safety feature working correctly. The underlying cause — reduced or absent airflow through the flue — is potentially serious if combustion were allowed to continue, as it could lead to incomplete burning and carbon monoxide production. Do not attempt to bypass the lockout; arrange a repair with a Gas Safe registered engineer promptly.