Navien E515 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E515 fault code mean?
The E515 code on a Navien boiler indicates an internal PCB (Printed Circuit Board) communication failure. In plain terms, one or more components inside the boiler have stopped communicating correctly with the main control board. Unlike some faults that clear themselves automatically, E515 requires a manual reset — the boiler will not restart on its own. Navien's service documentation identifies several sub-codes that sit beneath the E515 umbrella, each pinpointing a slightly different root cause: sub-codes 1–7 relate to a fault within the PCB's own internal circuitry (such as a resistance, transistor, or relay problem); sub-code 8 points to a communication break between the PCB and the ignitor; sub-code 9 indicates a PCB-to-fan communication issue; and sub-codes 10–12 signal abnormal communication between the PCB and the Dual Venturi assembly. If your display shows E515 followed by one of these numbers, share that sub-code with your engineer — it will help them diagnose the fault more quickly.
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 failing PCB internal circuit Common
The most frequent cause of a persistent E515. Components on the PCB itself — resistors, transistors, or relays — can degrade over time due to heat cycling, electrical surges, or simply age. When any of these internal elements fail, the board can no longer maintain reliable communication with the rest of the boiler. Sub-codes 1–7 specifically point here.
- Loose or damaged internal wiring connections Common
Vibration during normal operation, combined with the heat generated inside a boiler casing, can gradually loosen plug-in connectors between the PCB and components such as the ignitor (sub-code 8), fan (sub-code 9), or Dual Venturi (sub-codes 10–12). A connector that has worked loose can cause intermittent or permanent communication faults.
- Mains power supply instability or grounding noise Sometimes
Voltage fluctuations from the household electrical supply — particularly following storms, power cuts, or a shared circuit with high-demand appliances — can disrupt the PCB's internal communication. Electrical grounding noise on the supply can have a similar effect, corrupting signals between the board and connected components.
- Moisture or condensate ingress onto electronics Sometimes
A small leak from the heat exchanger, a blocked condensate trap, or high humidity inside the casing can allow moisture to settle on the PCB or its connectors. Even minor dampness can cause erratic signals or short circuits that trigger an E515.
- Fan malfunction or air intake blockage (NCB models) Rare
On certain Navien NCB combi models, a blocked air intake flue terminal or a combustion fan that is sticking or damaged can prevent the fan from reporting correctly to the PCB, triggering sub-code 9 within E515. Corrosion on the fan wiring connector is a known contributor.
How to fix it
- Power-cycle the boiler DIY safe
Switch the boiler off at the fused spur or isolation switch and leave it completely de-energised for at least 10 minutes. This allows any residual charge to dissipate and can clear a minor, one-off communication glitch. After 10 minutes, restore power and attempt a manual reset as per your Navien boiler's reset procedure. If E515 does not return within a normal heating cycle, monitor the boiler over the next day or two. Do not repeat this reset more than two or three times — a fault that keeps coming back needs professional attention.
- Check your external gas and electricity supplies DIY safe
Confirm that the gas supply to the boiler is fully open at the isolation valve and that other gas appliances in the home are working normally. Also check that the boiler's fused spur switch is in good condition and that no circuit breakers have tripped in your consumer unit. A disrupted power supply is a known trigger for E515 and is worth ruling out before calling an engineer.
- Inspect the flue terminal and air intake (NCB models) DIY safe
If you have a Navien NCB model, go outside and visually check that the flue outlet and air intake terminal are clear of obstructions — bird nests, debris, or ice in cold weather can restrict airflow. Do not attempt to dismantle any flue components. If you see an obvious external blockage such as a bird guard packed with leaves, clearing it is safe, but anything beyond that should be left to an engineer.
- Have an engineer inspect internal wiring and connectors Gas Safe engineer
A Gas Safe registered engineer should open the boiler casing and check all wiring loom connectors leading to and from the PCB. They will look for pins that have backed out of their housings, signs of heat damage or corrosion on connector blocks, and any wiring that has chafed against the casing. The sub-code displayed alongside E515 will guide the engineer to the specific circuit at fault — for example, sub-code 8 directs attention to the ignitor harness, sub-code 9 to the fan wiring, and sub-codes 10–12 to the Dual Venturi connection.
- Have the mains supply voltage and grounding checked Gas Safe engineer
An engineer will use a multimeter to confirm the boiler is receiving the correct mains voltage and that the supply is stable. They can also check whether electrical grounding noise on the incoming supply is interfering with the PCB's operation. If grounding noise is identified as the cause, an electrician may need to be involved to address the issue at the consumer unit.
- Replace the PCB if the internal circuit has failed Gas Safe engineer
If wiring and power supply checks come back clear, or if the fault matches sub-codes 1–7 indicating a PCB-internal failure, the control board will need to be replaced. This is a job for a Gas Safe registered engineer. Before authorising the repair, ask the engineer to check whether your boiler is still within Navien's manufacturer warranty period, as PCB faults resulting from a manufacturing defect may be covered at no cost to you.
- Contact a Gas Safe registered engineer Gas Safe engineer
If E515 returns after a power-cycle reset, or if you have any doubt about the boiler's safe operation, stop using the boiler and arrange for a Gas Safe registered engineer to carry out a full diagnostic. Continuing to run a boiler with an active PCB communication fault risks further component damage and higher repair costs down the line.
Parts you may need
- Navien PCB (main control board) · from £280
- Combustion fan assembly · from £120
- Ignitor assembly · from £65
- Internal wiring harness/loom · from £45
- Dual Venturi assembly · from £95
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–£550, depending on the underlying cause.
Frequently asked questions
Can I reset an E515 fault myself on a Navien boiler?
You can attempt a single power-cycle reset by switching the boiler off at the isolation switch for 10 minutes and then restoring power. If E515 clears and does not come back, a temporary glitch may have been the cause. However, if the code reappears — especially within the same heating cycle — do not keep resetting the boiler. A persistent E515 almost always means there is a genuine PCB or wiring fault that needs a Gas Safe engineer to investigate.
What does the sub-code number after E515 mean?
Navien uses sub-codes alongside E515 to narrow down which part of the communication circuit has failed. Sub-codes 1–7 indicate a fault within the PCB's own internal components (resistors, transistors, or relays). Sub-code 8 means the communication link between the PCB and the ignitor is broken. Sub-code 9 points to a PCB-to-fan communication issue. Sub-codes 10–12 indicate a fault in the communication between the PCB and the Dual Venturi assembly. Note the sub-code displayed on your boiler and pass it on to your engineer — it can save diagnostic time.
How much does it cost to fix a Navien E515 fault in the UK?
Most people with an E515 fault end up paying somewhere between £200 and £550 in total, depending on what the engineer finds. A wiring or connector repair at the lower end of that range might cost £150–£250 including the call-out. If the PCB itself needs replacing, the part alone typically costs £200–£350 for a Navien board, with labour on top bringing the total to £350–£550 or more. In rare cases where the PCB fault has caused wider damage, costs can exceed this — but a full boiler replacement (£1,800–£3,500) is only warranted when the boiler is old or has multiple failures. Always check your Navien warranty documentation first, as PCB faults within the warranty period may be covered.
Is it safe to keep using my Navien boiler when E515 is showing?
No. Navien themselves advise against running the boiler while E515 is active. A PCB communication failure can mean the boiler is not able to monitor or control its own components reliably, which could lead to unsafe operation or cause further damage to other parts. Switch the boiler off and arrange for a Gas Safe registered engineer to inspect it as soon as possible.