Navien E407 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E407 fault code mean?
The E407 code on a Navien boiler or water heater indicates a problem with the hot water outlet thermistor — the small sensor responsible for continuously monitoring the temperature of water as it leaves the unit. This thermistor feeds live temperature data to the PCB, allowing the boiler to regulate output accurately and safely. When the PCB detects that the thermistor signal is outside acceptable parameters — either because the sensor circuit has broken open or has shorted — it triggers E407 and locks the boiler out as a precaution. In short, the boiler has lost confidence in its ability to read outlet water temperature and has shut down rather than risk overheating.
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
- Open circuit on the outlet thermistor Common
The thermistor wiring has broken or the sensor element itself has failed internally, creating an open circuit. The PCB receives no signal at all and interprets this as a critical fault, triggering E407 and locking out the boiler. This is the single most common root cause of this code.
- Short circuit on the outlet thermistor Common
A damaged or degraded thermistor can fail by shorting internally, causing the resistance reading sent to the PCB to collapse. Equally, chafed or pinched wiring between the thermistor and the PCB can create a short that mimics a faulty sensor. The boiler cannot distinguish a valid low-temperature reading from a shorted signal, so it defaults to a lockout.
- Corrosion or moisture ingress on the wiring connector Sometimes
Condensation, water leaks, or poor sealing around the vent connection can allow moisture into the unit over time. This degrades the thermistor connector pins or the wiring itself, causing intermittent or permanently incorrect resistance readings. On the NPE-240A in particular, an incorrectly sealed escutcheon at the flue connection is a known contributor to internal moisture and associated sensor faults.
- General sensor wear and degradation Sometimes
Like all electronic components, thermistors have a finite lifespan. Years of thermal cycling — repeatedly heating and cooling — can cause the sensor element to drift out of calibration or fail gradually. This typically shows up on older units or those that have never been serviced, where the sensor reading becomes increasingly inaccurate before failing outright.
How to fix it
- Reset the boiler once or twice using the Back button on the front panel DIY safe
A single E407 event can occasionally be caused by a transient electrical glitch. Press the Back (reset) button on the boiler's front panel, allow the unit to restart, and check whether the fault clears. Do not reset more than two or three times in succession — repeated lockouts on the same code confirm an underlying fault and repeated resets will not resolve it.
- Check that the gas supply to the property is functioning normally DIY safe
Before assuming a component fault, confirm the gas supply is on at the meter and that other gas appliances in the property are working. A loss of gas supply can cause unusual boiler behaviour. If your gas supply is interrupted, contact your gas supplier rather than attempting any boiler work yourself.
- Inspect the area around the boiler for signs of water leaks or moisture DIY safe
Look around the boiler casing and the flue connection for any visible drips, watermarks, or corrosion staining. Moisture finding its way into the unit is a known trigger for sensor faults on Navien appliances, particularly around the flue escutcheon. Report any findings to the engineer — do not attempt to open the boiler casing yourself.
- Have a Gas Safe engineer inspect the hot water outlet thermistor and its wiring Gas Safe engineer
A qualified engineer will remove the boiler casing and use a multimeter to test the thermistor's resistance against Navien's published values at a known temperature. They will also check the wiring harness for continuity, check connector pins for corrosion or damage, and confirm the thermistor is correctly seated against the outlet pipe. This work must not be carried out by an unqualified person.
- Have the engineer replace the thermistor if it tests outside specification Gas Safe engineer
If the thermistor is confirmed faulty, the engineer will fit a compatible Navien replacement. A small amount of thermal paste should be applied between the sensor body and the pipe surface to ensure accurate heat transfer and reliable readings going forward. The engineer will then clear the fault code and run the boiler through a full test cycle to confirm normal operation.
- Ask the engineer to inspect and reseal the flue connection if moisture ingress is suspected Gas Safe engineer
On models such as the NPE-240A where an incorrectly fitted flue escutcheon has allowed condensate or outside air into the unit, the engineer should verify the seal is correct and address any corrosion on internal components. Ignoring the moisture source risks a repeat fault even after a new thermistor is fitted.
- If the fault persists after thermistor replacement, request PCB diagnostics Gas Safe engineer
In rare cases, the PCB itself develops a fault on the thermistor input circuit, causing E407 even with a healthy sensor. If the new thermistor tests correctly but the fault code returns, the engineer should perform PCB-level diagnostics. PCB replacement or repair is a specialist job and should only be undertaken by a Gas Safe registered engineer with Navien experience.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
E407 is not a fault that can be resolved by homeowner intervention beyond a reset. If the boiler has not cleared after one or two resets, contact a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk. Leaving the boiler locked out for extended periods in cold weather risks frozen pipes and additional damage.
Parts you may need
- Navien hot water outlet NTC thermistor · from £28
- Thermistor wiring harness / connector · from £22
- Thermal paste (for thermistor installation) · from £6
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 £120–£280, depending on the underlying cause.
Frequently asked questions
Can I fix the Navien E407 fault myself?
The only safe DIY steps are resetting the boiler once or twice using the Back button and checking for obvious moisture or leaks around the unit. Any inspection or replacement of the thermistor, its wiring, or any internal component must be carried out by a Gas Safe registered engineer. Working inside the boiler without the correct registration is illegal in the UK and could void your warranty or invalidate your home insurance.
How much does it cost to fix an E407 fault on a Navien boiler in the UK?
For a straightforward thermistor replacement during normal working hours, most UK homeowners pay in the region of £120–£280, which covers the call-out, labour, and the part itself. The thermistor component alone typically costs around £20–£35. If the fault turns out to involve the PCB rather than the thermistor, costs can rise considerably — PCB repair or replacement on a Navien unit can reach £400–£600 or more depending on parts availability — but this is an uncommon outcome for E407.
Why does my Navien boiler keep showing E407 after I reset it?
If E407 keeps returning after one or two resets, the thermistor or its wiring has a genuine electrical fault — either an open circuit, a short circuit, or corroded connector pins — that a reset cannot clear. Repeated resets simply delay an inevitable lockout. You should stop resetting and call a Gas Safe engineer to carry out a proper diagnosis with a multimeter, rather than running the boiler in a fault state.
Could a venting or condensate issue cause E407 on my Navien?
Yes, on certain Navien models — particularly the NPE-240A — an incorrectly fitted or poorly sealed flue connection can allow moisture and condensation to build up inside the unit. Over time this corrodes the thermistor connector and wiring, eventually triggering E407. If your engineer finds moisture or rust inside the boiler alongside a thermistor fault, ask them to check and properly reseal the flue escutcheon as part of the repair, otherwise the fault is likely to return.