Navien E205 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E205 fault code mean?
The E205 code appears when your Navien boiler detects a problem with the heating supply (flow) thermistor — the small temperature sensor fitted to the heating circuit's flow pipe. The boiler uses this sensor to monitor the water temperature being sent to your radiators or underfloor heating. When the sensor reads an implausible value — typically below -10°C, indicating an open circuit — or reports a short circuit, the boiler cannot safely regulate heating output and shuts itself down as a precaution. In short, the boiler has lost confidence in the temperature data from this sensor and refuses to operate until the issue is resolved.
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
- Failed or degraded thermistor Common
The heating supply thermistor itself has developed an open or short circuit — the most frequent trigger for E205. Thermistors are subject to wear over time, particularly in hard-water areas where scale build-up accelerates deterioration. Most sensors last around 5–7 years under normal use before reliability drops.
- Wet or corroded sensor connector Common
Moisture reaching the thermistor's electrical connector — from a nearby leak, condensation, or water ingress inside the casing — can cause erratic resistance readings that the boiler interprets as a sensor fault. Drying and cleaning the connector sometimes resolves the code if caught early.
- Damaged or loose wiring Sometimes
The wiring loom running from the thermistor to the PCB can develop breaks, chafing, or loose terminals over time, producing an apparent open circuit even when the sensor itself is healthy. A visual inspection and continuity check with a multimeter quickly identifies this.
- PCB (controller) fault Rare
In rare cases the PCB loses the ability to interpret the thermistor signal correctly, even when the sensor and its wiring test as good. PCB failure is a last-resort diagnosis made only after the sensor and wiring have been ruled out.
How to fix it
- Reset the boiler DIY safe
Press the Back button on the front panel to clear the fault and attempt a restart. If E205 disappears and does not return, monitor the boiler over the next day or two. If the code comes back — or does not clear at all — do not keep resetting repeatedly; move on and book an engineer.
- Check for an obvious leak near the sensor connector DIY safe
Without opening the boiler casing, look through the service panel (if accessible) or at any visible pipework around the boiler for signs of dripping water. A wet connector on the heating supply sensor can trigger E205. If you can see moisture, note it so you can tell the engineer — do not attempt to disconnect or dry connectors yourself.
- Confirm your gas supply is working DIY safe
Check that other gas appliances in your home (hob, gas fire) are working normally and that your gas isolation valve is fully open. While E205 is a sensor fault rather than a gas-supply fault, ruling out a general gas interruption takes moments and avoids an unnecessary call-out.
- Call a Gas Safe registered engineer to test the thermistor and wiring Gas Safe engineer
An engineer will use a multimeter to measure the thermistor's resistance across its operating temperature range and check continuity through the wiring harness. This confirms whether the sensor, the wiring, or the connector is at fault before any parts are ordered. Do not open the boiler casing or attempt to disconnect any components yourself.
- Replace the defective heating supply thermistor Gas Safe engineer
If testing confirms the sensor has failed, the engineer will source the correct replacement thermistor for your Navien model and fit it. They will then clear the fault, fire the boiler, and verify the temperature readings are within normal parameters before signing off the job.
- Replace the PCB if the fault persists after sensor replacement Gas Safe engineer
Should E205 return even after a confirmed thermistor replacement, the PCB (controller board) is likely misreading the signal. PCB replacement is a more involved and costly repair — your engineer will advise whether this is economical given the age of the boiler.
Parts you may need
- Navien heating supply thermistor (flow sensor) · from £25
- Thermistor connector/wiring harness · from £18
- Navien PCB / controller board · from £290
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–£300, depending on the underlying cause.
Frequently asked questions
Can I fix the Navien E205 code myself?
The only safe steps a homeowner can take are resetting the boiler once or twice and checking for obvious leaks near the boiler. Diagnosing and replacing the thermistor requires opening the boiler casing and working with electrical components — this must be done by a Gas Safe registered engineer. Navien themselves advise calling a qualified technician if the reset does not clear the code.
How much does it cost to fix a Navien E205 fault in the UK?
For the majority of E205 cases — where the thermistor itself is faulty — expect to pay roughly £120–£300 all in, covering the engineer's time and the sensor (typically £10–£40 for the part). If the PCB turns out to be the root cause, the board alone can cost £180–£400 in parts, pushing the total bill considerably higher. That scenario is much less common, so most people resolve E205 at the lower end of the range.
Why does my Navien boiler keep showing E205 after a reset?
A reset only clears the fault log — it does not fix the underlying problem. If the thermistor has genuinely failed, developed an open circuit, or its connector is wet, the same fault condition returns as soon as the boiler tries to fire. Repeated resets without a repair will not resolve E205 and could mask a worsening fault. Book an engineer to test the sensor properly.
How can I prevent thermistor faults on my Navien boiler?
Annual servicing is the best preventative measure — a Gas Safe engineer will check sensor readings, inspect connectors for corrosion or moisture, and identify early signs of wear before they cause a breakdown. In hard-water areas, fitting a scale reducer or magnetic filter protects sensors and the heat exchanger from limescale, which is a key contributor to premature sensor failure. Keeping boiler pressure between 1–1.5 bar also reduces stress on internal components.