Navien E445 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E445 fault code mean?
The E445 code appears on Navien boilers — most commonly the NPE series — when the control system detects that the bypass mixing valve is behaving abnormally. This valve normally regulates fluid flow and temperature by diverting a proportion of the flow around the mixing chamber when required. When its feedback sensor, motor, or physical mechanism falls outside expected parameters, the boiler locks out and displays E445 as a protective measure. Unlike some Navien faults, E445 requires a manual reset and will not clear on its own — pressing the Back button on the front panel is needed to attempt a restart.
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 bypass mixing valve Common
The valve itself — a piston-type component inside the boiler — can stick, seize, or fail in a partially open or closed position. A common symptom alongside E445 is a knocking or clunking noise at the end of a heating cycle as the unit drops back to standby, suggesting the valve mechanism is struggling. A valve stuck partially open during a hot water demand can also result in low flow rates or water that is hotter than expected at the tap.
- Failed mixing valve feedback sensor or wiring fault Common
The PCB monitors the mixing valve's position via a feedback sensor. If the sensor connector becomes loose, corroded, or the sensor itself fails, the board receives no reliable signal and triggers E445 even if the valve is physically intact. An engineer will check the wiring harness and connector before condemning the valve.
- Faulty mixing valve transformer Sometimes
A small transformer supplies power to drive the mixing valve motor. If this transformer fails, the valve cannot operate at all and the boiler interprets this as abnormal valve operation. It is worth an engineer checking the transformer output before replacing the valve assembly, as it may need replacing alongside or instead of the valve.
- Defective PCB (printed circuit board) Rare
In documented cases, replacing the bypass mixing valve alone did not clear the E445 fault, and only fitting a new PCB resolved it. The board may misread valve feedback signals or fail to send the correct drive signal, causing a persistent fault regardless of the valve's physical condition. PCB failure is the less likely root cause but should be considered when a new valve does not fix things.
How to fix it
- Attempt a manual reset via the front panel DIY safe
Press the Back button on the boiler's front panel to attempt a reset. E445 does not self-clear, so a manual reset is always required. If the code returns immediately or comes back after a short period of normal operation, do not keep resetting — repeated resets without addressing the underlying fault can cause further wear and will not resolve the problem.
- Check for any obvious external symptoms DIY safe
While the boiler is in fault state, take note of any unusual noises — particularly a knocking or banging sound that occurred at the end of the last heating cycle. Also note whether you experienced low hot water flow or unexpectedly high temperatures before the lockout. These observations help an engineer narrow down whether the valve is stuck, the sensor has failed, or the PCB is the culprit. Write them down before calling.
- Contact a Gas Safe registered engineer for diagnosis Gas Safe engineer
All internal inspection and repair work on a Navien boiler must be carried out by a Gas Safe registered engineer. The engineer will access the bypass mixing valve, check its physical condition for signs of seizure, blockage, or damage, test the feedback sensor and its wiring connector, and verify the transformer output. This diagnosis step is essential before any parts are ordered, as the valve, transformer, and PCB can all produce similar fault behaviour.
- Replace the bypass mixing valve if found to be defective Gas Safe engineer
The correct OEM replacement is the Navien bypass mixing valve (part number 30004837B for applicable NPE models — your engineer should verify compatibility with your specific unit). Be aware that on many Navien combi units, gaining access to the mixing valve requires removing a significant portion of internal components, including the PCB, control panel, internal pipework, and circulation pump. Labour time is therefore higher than on many other boilers, so a realistic quote should reflect this.
- Inspect and replace the PCB if the fault persists after a new valve Gas Safe engineer
If E445 returns after the mixing valve and transformer have been replaced, the PCB is the most likely remaining cause. A Gas Safe engineer should test the board's outputs before recommending replacement. PCB replacement on Navien units can be a significant cost — see the FAQ below — so confirm the diagnosis is solid before proceeding.
Parts you may need
- Navien bypass mixing valve (30004837B) · from £120
- Mixing valve transformer · from £45
- Navien PCB / main control board · from £280
- Feedback sensor / wiring harness · from £35
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 £200–£450, depending on the underlying cause.
Frequently asked questions
Can I reset the Navien E445 myself?
Yes — pressing the Back button on the front panel is the correct reset method and is safe for a homeowner to do. However, E445 is a lockout fault that will not go away on its own, and resetting it without an underlying repair is only a temporary measure. If the code returns after one or two resets, stop resetting and call a Gas Safe engineer, as repeated cycling without a fix will not resolve the fault and could mask a worsening problem.
How much does it cost to fix a Navien E445 fault in the UK?
For most households, a bypass mixing valve replacement — the most common repair for E445 — comes to roughly £200–£450 all in, including the engineer's call-out, labour, and the part. Navien boilers require significant strip-down to access the mixing valve, so labour time is higher than average. If the PCB also needs replacing, add a further £180–£400 for the part alone, pushing total costs toward £500–£700 or beyond. If your boiler is over 10–12 years old and facing a repair bill of this size, it is worth getting a new boiler quote for comparison.
Why does my Navien make a knocking noise and then show E445?
A knocking or clunking sound at the end of a heating cycle — just as the boiler returns to standby — followed by an E445 lockout is a recognised symptom of a failing bypass mixing valve. The noise typically comes from the valve mechanism struggling to return to its rest position. This pattern strongly suggests the valve itself rather than a sensor or PCB issue, though your engineer will still verify the wiring and transformer before ordering parts.
Is E445 on a Navien boiler a Gas Safe job or can a competent person fix it?
All repair work inside a gas boiler in the UK must be carried out by a Gas Safe registered engineer — this is a legal requirement, not just a recommendation. Working on gas appliances without registration is illegal, will void your boiler warranty and home insurance cover, and carries serious safety risks. You can safely reset the boiler yourself and note down symptoms, but everything beyond that must be left to a registered professional. You can find Gas Safe engineers at the official Gas Safe Register website (gassaferegister.co.uk).