Navien E353 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E353 fault code mean?
The E353 code on a Navien boiler signals an abnormal reading from the water pressure sensor. The boiler's control board is either receiving no signal, an out-of-range signal, or an erratic reading from the sensor responsible for monitoring system water pressure. As a safety measure, the boiler shuts itself down completely — cutting both heating and hot water — until the fault is resolved. On some UK-installed Navien NCB combi models, the display may also show an 'air' indicator alongside E353, reflecting the fact that air trapped in the heat exchanger or pipework can prevent the sensor from reading correctly. Both symptoms ultimately point to the same root issue: the boiler cannot confirm that water pressure is within a safe operating range.
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 water pressure sensor Common
The sensor itself may have developed an open or short circuit internally, causing it to send a corrupt or absent signal to the PCB. This is the most direct cause of E353 and typically requires sensor replacement by an engineer.
- Air trapped in the heat exchanger or system pipework Common
Air pockets prevent the sensor from reading accurate water pressure, often triggering E353 alongside an 'air' indicator on the display. Bleeding radiators and purging air from the system can resolve this, but if air keeps returning it may indicate a deeper fault such as a small leak or a failing auto air vent.
- Loose wiring or corroded connector at the sensor Common
Vibration and moisture over time can cause the wiring harness connected to the pressure sensor to work loose or corrode at the connector pins. This produces an intermittent or absent signal that the boiler interprets as a sensor fault.
- Moisture or leakage around the sensor Sometimes
The Navien NCB service manual specifically flags moisture on the cold-water-side pressure sensor as a trigger for E353. A nearby micro-leak can wet the sensor body or its connector, causing false readings or short circuits.
- Excessive water pressure spikes Sometimes
Sudden pressure spikes above the sensor's rated range — sometimes caused by a faulty expansion vessel, a failed pressure relief valve, or mains supply surges — can cause the sensor to lock out the boiler as a protective response.
- Debris or sediment blocking the sensor port Rare
Rust, limescale, or other debris in the system water can partially block the pressure sensor's port, causing a dampened or inaccurate reading. This is more common in older or poorly maintained systems that have never been chemically flushed.
How to fix it
- Check system pressure on the boiler's display or pressure gauge DIY safe
Navien recommends a system pressure of around 1–1.5 bar when the boiler is cold. If the gauge reads below 1 bar, the pressure sensor may be reporting a low-pressure fault rather than a true sensor failure. Top up the pressure using the filling loop (usually located beneath the boiler) until the gauge sits at around 1.2 bar, then re-check.
- Bleed your radiators to remove trapped air DIY safe
Starting with the radiator furthest from the boiler, use a radiator bleed key to open each bleed valve slightly. Hold a cloth beneath the valve and wait until a steady stream of water appears rather than air or a spluttering mixture, then close the valve. Work your way towards the boiler. After bleeding, recheck system pressure and top up if needed.
- Reset the boiler DIY safe
Once pressure and air have been addressed, press the Back or Reset button on the Navien control panel to clear the E353 code. If the boiler fires successfully, monitor it over the next 24–48 hours to see whether the fault returns. Limit resets to two or three attempts — repeated lockouts without resolution suggest a component fault that needs professional attention.
- Inspect the sensor wiring and connectors (engineer task) Gas Safe engineer
A Gas Safe registered engineer should visually check the pressure sensor's wiring harness for chafing, corrosion, or loose connector pins. Resistance and continuity tests across the sensor circuit will confirm whether the wiring or the sensor itself is the source of the fault.
- Check the sensor body and port for moisture or leaks Gas Safe engineer
The engineer should examine the sensor and surrounding pipework for any sign of dampness or active weeping. If the sensor or its connector is wet, the leak source must be traced and repaired before fitting a replacement sensor, otherwise the new component may be damaged immediately.
- Test and replace the water pressure sensor if required Gas Safe engineer
If wiring tests and visual checks do not reveal the issue, the engineer will test the sensor's output signal directly. A faulty sensor should be replaced with a genuine Navien-compatible part. Following replacement, the system should be fully purged of air before firing the boiler, as residual air pockets can trigger a repeat of E353.
- Assess system condition for pressure spikes or debris Gas Safe engineer
If the sensor and wiring are in good condition yet E353 persists, the engineer should check the expansion vessel pre-charge pressure and inspect the pressure relief valve for evidence of frequent activation. A power flush or chemical system cleanse may be recommended if significant sludge or debris is found in the system.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
If the boiler continues to display E353 after the homeowner checks (pressure top-up, bleeding, reset), contact a Gas Safe registered engineer. You can verify any engineer's credentials at the Gas Safe Register website (gassaferegister.co.uk). Do not continue resetting the boiler repeatedly, as this will not resolve an underlying sensor or wiring fault.
Parts you may need
- Navien water pressure sensor · from £25
- Wiring harness / connector for pressure sensor · from £18
- Expansion vessel (if pressure spikes are the cause) · from £55
- Auto air vent (AAV) · from £12
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 E353 myself without calling an engineer?
A few basic checks are safe for a homeowner: topping up system pressure to around 1–1.2 bar via the filling loop, bleeding radiators to remove trapped air, and resetting the boiler up to two or three times. If E353 clears and stays away, great. If it keeps returning, or if you have any doubt about what you are doing, you need a Gas Safe registered engineer. Replacing the pressure sensor, testing wiring, or inspecting internal components are all jobs that require professional expertise and, by law, must be carried out by a suitably qualified person.
How much does it cost to fix Navien E353 in the UK?
For most households the total bill — parts and labour — lands somewhere between £120 and £320. The pressure sensor itself is a relatively inexpensive component at around £15–£30, so the main cost is the engineer's labour time (typically £60–£100 per hour). Simple cases where only a bleed and reset are needed will sit at the lower end; if the sensor, wiring harness, or expansion vessel also need attention, costs rise towards the higher end of that range. Emergency call-outs or work carried out in the evenings and at weekends can push the total higher — sometimes £250–£400 — due to premium call-out rates.
Why does my Navien boiler show E353 alongside an 'air' indicator?
Some UK Navien NCB combi-boiler models display an 'air' warning at the same time as E353. This combination suggests that air is trapped in the heat exchanger or system pipework, preventing the pressure sensor from reading correctly. The boiler interprets the absence of a consistent pressure signal as a sensor fault and locks out. Bleeding the radiators, checking system pressure, and resetting will often clear both indicators. If the air indicator keeps returning after bleeding, there may be a small leak introducing air into the system, or the auto air vent may be stuck — both of which require an engineer to investigate.
Is E353 on a Navien boiler dangerous?
E353 itself is not a gas-safety emergency in the way that a carbon monoxide alarm or a gas smell would be. It is a sensor fault that causes the boiler to shut down safely rather than run at an unknown pressure. However, you should not repeatedly override the lockout by resetting without addressing the underlying cause, and you should not attempt to bypass or remove the pressure sensor yourself. If at any point you smell gas or suspect a carbon monoxide leak, leave the property immediately, open windows, and call the National Gas Emergency Service on 0800 111 999.