Navien E594 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E594 fault code mean?
The E594 code on a Navien boiler signals either a partial PCB communication breakdown or an EEPROM fault. Put simply, the main control board is struggling to communicate correctly with one or more connected components, or the small memory chip on the board (the EEPROM, which stores configuration data) has developed a problem. Navien uses sub-codes alongside E594 to narrow down exactly where the fault lies — always note the sub-code number shown on your display, as it tells you which component is at fault: Sub-code 0 = high-limit switch of the heat exchanger (check PCB); Sub-code 1 = exhaust sensor wiring or sensor itself; Sub-code 2 = flame rod connection or flame sensor; Sub-codes 3–14 = PCB fault (check the PCB); Sub-code 15 = pressure sensor reading below expected range (check wiring and output voltage); Sub-code 16 = pressure sensor reading above expected range (check wiring and output voltage). Sharing your sub-code with an engineer before they arrive will save diagnostic time and potentially reduce the call-out cost.
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
- PCB failure or EEPROM fault Common
The most frequent root cause of E594 — particularly sub-codes 0 and 3–14 — is the PCB itself developing a communication fault or the EEPROM memory chip losing its stored configuration. This can happen through age, power surges, or component degradation, and typically requires professional diagnosis and board replacement or reconfiguration.
- Moisture or water ingress on the PCB Common
Even minor water vapour or a slow internal leak reaching the PCB can cause corrosion on solder joints and tracks, triggering intermittent or sustained communication errors. Check for visible drips or damp patches around the boiler casing — if found, turn the boiler off and call an engineer without delay.
- Loose or corroded wiring connections Common
A loose connector on the exhaust sensor (sub-code 1), flame rod (sub-code 2), or pressure sensor (sub-codes 15–16) can mimic a component failure by breaking the signal path. Vibration over time or a previous repair where connectors were not fully reseated are common triggers.
- Faulty exhaust sensor Sometimes
Sub-code 1 specifically points to the exhaust temperature sensor. If the sensor itself has failed or its wiring has chafed or corroded, the PCB cannot receive a valid reading and flags E594. An engineer will test the sensor's resistance and inspect its wiring loom.
- Flame rod fault Sometimes
Sub-code 2 indicates the flame rod or its associated wiring is not behaving as expected. The flame rod detects whether a flame is present; if its signal is interrupted or out of range, the PCB raises a communication fault. Carbon build-up on the rod or a cracked ceramic insulator are typical culprits.
- Pressure sensor out of range Sometimes
Sub-codes 15 and 16 point to the water pressure sensor reporting a value below or above its expected output voltage range. This may be a failing sensor, a wiring issue, or — in sub-code 16 — genuine overpressure that has damaged the sensor. The engineer will check both the sensor output and the system pressure.
- PCB misconfiguration after replacement Rare
If the PCB has been replaced recently and E594 appears, the new board may not have been correctly configured or synchronised to the boiler's settings. This is a known issue with freshly installed boards and is resolved by the engineer entering the appropriate configuration codes — not a hardware fault as such.
How to fix it
- Note the sub-code displayed alongside E594 DIY safe
Before doing anything else, write down the full code as shown on the boiler display — for example 'E594-1' or 'E594-15'. This sub-code tells you (and any engineer you call) exactly which component the PCB is flagging, cutting down diagnostic time significantly.
- Attempt a power reset DIY safe
Switch the boiler off at the power switch or unplug it from the mains socket. Wait a full 30 seconds to allow the internal electronics to fully discharge, then restore power. A genuine one-off communication glitch will sometimes clear with a reset. If E594 returns immediately or after a short time, do not keep resetting — move on to the next step.
- Check for visible moisture or leaks around the boiler DIY safe
Look carefully underneath and around the boiler casing for drips, damp patches, or water staining. Even a small internal leak can cause PCB damage if left unchecked. If you find any signs of water, turn the boiler off at the mains and do not restart it until an engineer has assessed it.
- Check system pressure (if sub-code 15 or 16 is showing) DIY safe
If your display shows E594-15 or E594-16, glance at the pressure gauge on your boiler. Normal operating pressure for most Navien combi systems is between 1.0 and 1.5 bar when cold. If pressure is very low (below 0.5 bar), you can top it up via the filling loop following your boiler manual. If pressure is normal or high, leave it alone and report the reading to the engineer.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
All internal investigation — including inspecting the PCB, testing the exhaust sensor, checking the flame rod, measuring sensor output voltages, and replacing any components — must be carried out by a Gas Safe registered engineer. The engineer will use a multimeter to test voltage and continuity across the PCB, inspect wiring looms and connectors, and determine whether the board needs reconfiguration, repair, or full replacement. Do not open the boiler casing or attempt to disconnect any internal components yourself.
Parts you may need
- Navien PCB (main control board) · from £280
- EEPROM memory chip (PCB-mounted) · from £25
- Exhaust temperature sensor · from £55
- Flame rod assembly · from £45
- Water pressure sensor · from £60
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 £150–£500, depending on the underlying cause.
Frequently asked questions
What does the sub-code next to E594 mean on my Navien boiler?
Navien displays a sub-code alongside E594 to pinpoint the fault. Sub-code 0 relates to the heat exchanger high-limit switch; sub-code 1 to the exhaust sensor; sub-code 2 to the flame rod; sub-codes 3–14 all point to the PCB itself; sub-code 15 means the pressure sensor is reading below its expected range; and sub-code 16 means it is reading above range. Always tell your engineer which sub-code you see — it can save them 30–60 minutes of diagnostic time.
Can I fix Navien E594 myself?
The only safe DIY steps are noting the sub-code, performing a single power reset, checking for visible leaks, and (for sub-codes 15 or 16) checking the system pressure gauge. Everything beyond that — opening the boiler casing, testing the PCB, replacing sensors or the flame rod — requires a Gas Safe registered engineer. Attempting internal repairs yourself is both dangerous and illegal under UK gas safety regulations.
How much does it cost to fix E594 on a Navien boiler?
For sensor-related sub-codes (exhaust sensor, flame rod, or pressure sensor), repairs typically cost £150–£300 including parts and labour. If the PCB itself needs replacing, expect to pay £300–£500 for most Navien models in 2025, though a minor repair such as reflowing a solder joint may come in closer to £150–£200. In exceptional cases — for example where water damage has also affected surrounding wiring — costs can climb higher. Navien offers manufacturer-backed warranties of up to seven years, so check whether your boiler is still covered before authorising any paid repair.
My Navien PCB was recently replaced and now E594 is showing — why?
This is a recognised issue with newly fitted Navien PCBs. After replacement, the board must be configured and synchronised to the specific boiler model using setup codes. If this step was skipped or done incorrectly, E594 can appear as a memory or configuration fault rather than a hardware failure. Contact the engineer who carried out the replacement — this should be resolved under their warranty of works at no additional charge to you.