Navien E787 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E787 fault code mean?
The E787 code appears on Navien boilers when a manual reset attempt has not completed successfully. Rather than indicating a specific component failure in its own right, it flags that something interrupted or prevented the reset cycle from finishing cleanly. In many real-world cases — particularly on Navien NCB combi boilers — E787 clears briefly and then gives way to a second, underlying fault code such as E003 (an ignition or pressure-related fault). This means E787 is often best understood as a signal that something deeper needs attention, rather than a standalone problem. The good news is that a careful power cycle resolves it in a fair number of cases at no cost at all.
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
- Interrupted or incomplete power cycle Common
If the boiler was switched back on too quickly after being turned off, or if there was a brief power fluctuation during the reset attempt, the control board may not have completed its self-check routine, triggering E787. This is the most straightforward cause and is easily resolved by performing a proper, deliberate power cycle.
- Underlying fault preventing a clean reset Common
In many reported UK cases, E787 disappears after a reset but is immediately replaced by another code — most often E003. This pattern suggests the E787 itself was caused by an unresolved fault (such as low system pressure, an ignition failure, or a flow issue) that the boiler was trying to communicate before the reset could complete. Always check whether a second code follows.
- Loose or corroded PCB connections Sometimes
The control board relies on secure electrical connections to sensors, actuators, and wiring harnesses throughout the boiler. If any of those connections are loose, corroded, or damaged, the board may fail to complete a reset sequence reliably and throw E787 repeatedly. Codes in the nearby E78x range — including E782 — are directly linked to PCB communication issues, and E787 shares that territory.
- Faulty PCB or control board Rare
If all connections are sound but the reset keeps failing, the control board itself may be malfunctioning. A degraded or partially failed PCB can struggle to execute its reset routine correctly, producing E787 even when no other obvious fault is present. This is a less common cause but becomes more likely when simpler checks have already been ruled out.
How to fix it
- Turn the boiler off at the mains and leave it for at least 30 seconds DIY safe
A full power cycle — not just a button press — gives the control board time to fully discharge and restart its initialisation sequence. Switch off at the isolator or plug, wait a minimum of 30 seconds, then restore power. This alone resolves E787 in a good number of cases.
- Alternatively, use the boiler's built-in reset button DIY safe
On most Navien models, pressing and holding the Back or Reset button on the front panel for a few seconds will attempt a soft reset. If the display clears and the boiler fires up normally, monitor it over the next hour or so to confirm the fault does not return.
- Check whether a second fault code appears after E787 clears DIY safe
If E787 disappears but another code — for example E003 — immediately takes its place, that secondary code is the real fault to address. Note down whatever appears and look it up separately. E003 on Navien boilers is commonly linked to ignition failure or low system pressure.
- Check your system pressure and top up if needed DIY safe
Look at the pressure gauge on the boiler. It should sit between 1.0 and 1.5 bar when the system is cold. If it reads below 1 bar, use the filling loop (usually a braided flexible hose with two valves beneath the boiler) to slowly top up to around 1.2–1.3 bar, then close both valves. If you are unsure how to do this, your boiler's quick-start guide will show the filling loop location.
- Do not reset the boiler more than two or three times if the fault keeps returning DIY safe
Repeated resets without the fault clearing can mask a worsening problem and should be avoided. If E787 comes back after two or three attempts, or if it is consistently followed by another error code, it is time to call in a professional rather than continuing to cycle power.
- Have a Gas Safe engineer inspect the PCB connections and internal wiring Gas Safe engineer
If the reset keeps failing and no obvious cause has been found, the problem is likely inside the boiler — loose connectors on the PCB, damaged wiring, or a faulty control board itself. A Gas Safe registered engineer can safely open the casing, inspect all connections for corrosion or poor contact, and test the board with the correct diagnostic tools.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
Any internal repair to a gas boiler in the UK must be carried out by a Gas Safe registered engineer. If the steps above have not resolved E787, or if you are not confident identifying a secondary fault code, book an engineer and tell them the code — including any code that followed it — so they can prepare accordingly.
Parts you may need
- Navien PCB / main control board · from £220
- Pressure sensor · from £35
- Flow sensor · from £45
- Wiring harness · from £55
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 £80–£400, depending on the underlying cause.
Frequently asked questions
Can I fix Navien E787 myself?
The initial step — a proper power cycle at the mains — is perfectly safe for a homeowner to carry out, and it resolves the fault in a fair number of cases for free. Checking system pressure and topping up via the filling loop is also a DIY-safe task. Beyond that, any investigation inside the boiler casing, including checking PCB connections or replacing components, must be done by a Gas Safe registered engineer.
Why does E787 clear but then another code like E003 appears straight away?
This is actually a useful clue. It means the E787 was caused by an underlying fault that prevented the reset from completing cleanly. Once the reset does go through, the boiler immediately detects that underlying problem and reports it as a new code. E003 on Navien boilers typically points to an ignition failure or a pressure issue. Note the secondary code down — that is the fault you or your engineer need to fix.
How much does it cost to repair a Navien boiler showing E787?
If a simple power cycle clears the fault, the cost is nothing. If an engineer is needed for diagnostics, expect to pay £80–£150 for a callout and inspection. Replacing a pressure or flow sensor typically adds £100–£200 in parts and labour. A PCB replacement is the more expensive scenario, usually falling in the £250–£400 range including parts and fitting. Very extensive electrical faults or heat exchanger problems can cost more, but those are uncommon outcomes for this particular code.
Is Navien E787 a dangerous fault?
Not directly — it is a control and reset-process error rather than a gas leak or combustion safety alert. The boiler will typically lock out rather than continue operating in an unsafe state. That said, you should not keep resetting it repeatedly if it keeps returning, as this could mask a more serious underlying fault. If you smell gas at any point, do not attempt any resets — leave the property, open windows, and call the National Gas Emergency Service on 0800 111 999.