Navien E127 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E127 fault code mean?
The E127 code on Navien boilers and hot water units indicates a problem with the Air Pressure Sensor (APS) — either an open-circuit or short-circuit fault within the sensor itself. In practice, particularly on NPN-series units, this code also surfaces when the appliance loses its flame during operation. The two are closely linked: if the combustion fan fails to reach the correct speed, the APS cannot confirm adequate airflow, so the control board aborts the ignition sequence and locks the unit out. The typical homeowner symptom is a complete loss of hot water with the boiler refusing to fire 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
- Faulty or shorted Air Pressure Sensor (APS) Common
The APS monitors airflow through the combustion chamber. If the sensor develops an open or short circuit, it sends an invalid signal to the control board, which immediately locks the unit out with E127. This is the primary manufacturer-defined cause of this code.
- Combustion fan not reaching speed Common
When the boiler attempts to fire, you may hear it hum briefly before locking out. This is the fan trying to spin up. If the fan motor is worn or seized and cannot reach its target RPM within a couple of seconds, the APS never sees the expected airflow and the unit aborts. A failed fan is one of the most frequently diagnosed hardware faults behind this code.
- Dirty or degraded flame sensor Sometimes
The flame sensor rod can accumulate a layer of soot or oxidisation over time, preventing it from reliably detecting the presence of a flame. The unit may ignite briefly but shut down moments later, triggering E127 as a flame-loss event.
- Blocked or restricted flue or intake pipe Sometimes
Navien units typically use concentric or twin PVC pipes for combustion air intake and exhaust. A blockage from debris, a bird nest, or ice during cold weather can hold the APS in a closed position regardless of fan operation, causing E127 to appear even if all internal components are healthy.
- Gas supply or gas valve issue Sometimes
If the gas supply pressure is low or the gas valve is not opening correctly, the burner cannot sustain a flame and the unit locks out. This cause is less common but should be ruled out, especially if other gas appliances in the property are also behaving unusually.
- Wiring fault or PCB failure Rare
Damaged wiring to the APS or fan, loose connectors, or a fault on the control board itself can produce the same E127 error even when all the physical components are in good condition. This is typically diagnosed after other causes have been ruled out.
How to fix it
- Reset the boiler DIY safe
Press the Back button on the Navien front panel, or power the unit off at the isolator for 30 seconds before switching it back on. Attempt a reset no more than two or three times. If the fault clears and stays away, monitor the unit. If E127 returns promptly, move on to the checks below rather than continuing to reset.
- Confirm your gas supply is active DIY safe
Check whether other gas appliances in your home — a gas hob, gas fire, or another boiler — are working normally. If they are not, contact your gas supplier rather than an engineer, as the issue may be with the incoming mains supply. If other appliances work fine, the supply to the Navien is likely adequate.
- Check the flue and air intake pipes for blockages DIY safe
Go outside and locate the PVC exhaust and intake pipes on the external wall. Look for obvious obstructions such as debris, ice, a bird nest, or anything pressed up against the pipe terminals. Clear any superficial blockage if it is safe to do so. Do not attempt to dismantle or modify the flue pipework.
- Check whether the unit is under warranty DIY safe
Navien offers a five-year parts warranty on many components. If your unit is relatively new, contact Navien UK or your installer before booking an independent repair — you may be entitled to a free or subsidised fix. Have your serial number and installation date to hand.
- Do not attempt to inspect or replace the APS, fan, flame sensor, or gas components yourself Gas Safe engineer
Internal components including the Air Pressure Sensor, combustion fan, flame sensor rod, igniter, gas valve, and PCB must only be inspected, cleaned, or replaced by a Gas Safe registered engineer. Working on these parts without Gas Safe registration is illegal and potentially dangerous.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
A qualified engineer will test the APS electrically to determine whether it is open or shorted, check fan RPM feedback, inspect and clean or replace the flame sensor, verify gas valve operation, and test the PCB and wiring. Most E127 faults are resolved by replacing the APS or servicing the combustion fan — both straightforward jobs for a competent engineer. Always ask to see the engineer's Gas Safe ID card before work begins.
Parts you may need
- Navien Air Pressure Sensor (APS) with hoses · from £95
- Combustion fan motor · from £130
- Flame sensor rod · from £25
- Igniter assembly · from £45
- Control PCB (model-specific) · from £220
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 £150–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Navien E127 code myself?
The only safe DIY steps are resetting the boiler (up to two or three times), confirming your gas supply is on, and checking the external flue pipes for obvious blockages. Any internal work — testing or replacing the APS, flame sensor, fan, gas valve, or PCB — must be carried out by a Gas Safe registered engineer. Attempting this yourself is illegal and dangerous.
How much does it cost to fix a Navien E127 fault in the UK?
Most homeowners pay between £150 and £320 for the typical repairs behind E127. Replacing the Air Pressure Sensor (parts plus labour) usually comes to around £175–£250. Cleaning or replacing the flame sensor is generally £100–£200. Fan motor work tends to fall in the £150–£300 range. If the control board (PCB) turns out to be the culprit — which is less common — costs can reach £350–£450, but an engineer will usually exhaust cheaper options first.
Why does my Navien boiler hum for a second and then show E127?
That brief hum is the combustion fan attempting to spin up to its target speed. If the fan cannot reach the correct RPM — due to a worn motor, seized bearings, or a wiring fault — the Air Pressure Sensor never detects the expected airflow and the control board aborts the start sequence within a couple of seconds, displaying E127. A Gas Safe engineer will need to test the fan and APS to identify which component has failed.
My Navien is relatively new — could E127 be covered by warranty?
Quite possibly. Navien offers a five-year parts warranty on many components across their range. If your unit is within the warranty period, contact Navien UK customer support or your original installer before booking an independent repair. You will need your serial number and proof of the original installation date. Even if labour is not covered, having parts supplied under warranty can significantly reduce the bill.