Navien E218 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E218 fault code mean?
The E218 fault code on a Navien boiler indicates a problem with the heating return thermistor — the temperature sensor that monitors the temperature of water flowing back into the boiler from your central heating circuit (radiators or underfloor heating). Specifically, the boiler's control system has detected either an open circuit (meaning the sensor or its wiring is broken or disconnected) or a short circuit (meaning there is an electrical fault causing the sensor signal to collapse). Because the boiler can no longer reliably measure return water temperature, it shuts down as a safety precaution. This fault requires a Gas Safe registered engineer to diagnose and resolve.
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 failed heating return thermistor Common
Over time, the thermistor itself can degrade internally — the resistive element inside the sensor breaks down, causing it to report an open or short circuit to the PCB. This is the most frequent root cause and typically requires a straightforward sensor replacement by an engineer.
- Loose or damaged wiring connections Common
The wiring harness connecting the heating return thermistor to the PCB can develop loose pins, corroded terminals, or chafed insulation — any of which can mimic a failed sensor. An engineer will check the connector block at both the thermistor end and the board end before condemning the sensor itself.
- Short circuit in the wiring loom Sometimes
Wiring routed close to hot components or pinched during previous servicing can develop insulation breakdown, creating a short circuit. This would cause the boiler to read the thermistor as shorted even if the sensor itself is in perfect condition.
- PCB fault Rare
If the thermistor and all associated wiring test correctly with a multimeter, the fault may lie within the PCB's sensor input circuit. A board fault is relatively uncommon but possible, particularly on older Navien units or those that have experienced electrical surges.
How to fix it
- Reset the boiler once or twice DIY safe
Press the Back button on the Navien front panel to attempt a reset. If the E218 code clears and the boiler runs normally, monitor it closely over the next few hours. If the fault returns promptly or you have already reset it two or three times without success, do not keep resetting — move on to the next steps and call an engineer. Repeated resets without resolving the underlying fault can put additional stress on components.
- Check that the boiler's system pressure is within range DIY safe
While the thermistor fault itself is not pressure-related, it is worth confirming the pressure gauge reads between 1 and 1.5 bar before calling an engineer. If it is below 1 bar, top it up via the filling loop. This rules out any secondary issues and gives the engineer a clean starting point.
- Contact a Gas Safe registered engineer to inspect the heating return thermistor Gas Safe engineer
The engineer will locate the heating return thermistor — typically fitted in the return pipe close to the heat exchanger — and use a multimeter to measure its resistance at a known temperature. A reading outside the expected range confirms the sensor has failed. They will also inspect the wiring harness and connector blocks for signs of corrosion, damage, or poor contact.
- Engineer to repair or replace wiring if a connection fault is found Gas Safe engineer
If the thermistor itself tests within spec but the wiring shows damage, loose terminals, or corrosion, the engineer will repair or replace the affected section of loom and retest. This is often a quicker and cheaper resolution than a full sensor replacement.
- Engineer to replace the heating return thermistor if confirmed faulty Gas Safe engineer
Navien thermistors are specific to the model range. The engineer will fit the correct replacement sensor, ensure it is properly seated and connected, then run the boiler through a full heating cycle to confirm the E218 code has cleared and temperatures are being read accurately.
- Engineer to assess the PCB if the fault persists after thermistor and wiring checks Gas Safe engineer
If both the sensor and wiring are confirmed healthy but E218 continues to appear, the PCB's sensor input circuit may be at fault. The engineer will advise on whether a board repair or replacement is the most cost-effective route given the age of the boiler.
Parts you may need
- Navien heating return thermistor (NTC sensor) · from £25
- Thermistor wiring harness / connector loom · from £30
- Navien PCB (control board) · from £250
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–£320, depending on the underlying cause.
Frequently asked questions
Can I fix Navien E218 myself?
Not safely, beyond a reset attempt. The actual diagnosis — testing the thermistor with a multimeter and inspecting internal wiring — requires accessing components inside the boiler casing, which should only be done by a Gas Safe registered engineer. Attempting this yourself could invalidate your warranty and, if wiring is disturbed incorrectly, create a safety hazard.
How much does it cost to fix Navien E218 in the UK?
For most homeowners the repair comes to between £120 and £320 all in, covering the engineer's call-out, labour, and a replacement thermistor or wiring repair. If the PCB turns out to be the culprit, a replacement board for a Navien boiler typically costs £200–£300 for the part alone, which can push the total bill higher — something worth discussing with your engineer before authorising the work.
Will E218 keep coming back after a reset?
If the underlying fault — a failed sensor, damaged wiring, or a PCB issue — has not been fixed, yes, E218 will return shortly after each reset. Resetting repeatedly without a proper repair can also put extra wear on other components. One or two resets to confirm the fault is persistent is fine, but after that the boiler needs a professional inspection.
How can I avoid thermistor faults on my Navien boiler?
An annual service by a Gas Safe engineer is the single most effective measure. During a service, the engineer will check sensor readings, inspect wiring and connections, and catch early signs of deterioration before they cause a lockout. Keeping your boiler serviced also keeps the manufacturer warranty valid, which could cover the cost of some component failures.