Navien E432 Fault Code: Causes, Fixes & Repair Costs
What does the Navien E432 fault code mean?
The E432 fault code on Navien boilers and water heaters (commonly seen on NPE-series units) signals that the cold water inlet 2 thermistor has developed an open or short circuit. This small temperature-sensing component monitors the temperature of cold water entering the second inlet on the unit. When its resistance or voltage readings fall outside the expected range — whether because of a failed sensor, wiring damage, or a loose connection — the boiler's controls detect the fault and display E432 to protect the system. Note that the closely related code E421 refers to the cold water inlet 1 thermistor; if you see E421 instead, the fault lies with the first sensor rather than the second.
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 shorted thermistor Common
The cold water inlet 2 thermistor itself has failed internally, either developing an open circuit (no conductivity) or a short circuit (resistance collapses to near zero). Both conditions cause the sensor to report implausible readings, triggering E432. This is the most frequent root cause.
- Damaged or frayed wiring Common
The wiring harness connecting the thermistor to the PCB can develop breaks, fraying, or chafing — often in areas where cables are routed close to pipework or sharp edges. Even a partial break in the wire can cause intermittent or permanent faults.
- Loose or disconnected connector Sometimes
The plug connecting the thermistor to the control board may have worked loose over time due to vibration or previous servicing. A poorly seated connector can mimic a thermistor failure without the sensor itself being damaged.
- Electrical supply irregularities Rare
Unstable mains voltage or a poor earth connection can occasionally cause false E432 readings. If the fault clears after a reset and does not return, this may be the underlying cause, but it should still be investigated by an engineer.
How to fix it
- Reset the boiler and observe whether the fault returns DIY safe
Turn the boiler off at its power switch, wait around 30 seconds, then power it back on. If E432 disappears and does not reappear within a short period of normal operation, the fault may have been a transient electrical glitch. If the code returns promptly or repeatedly, move on to the next steps.
- Check that the boiler's electrical supply is stable DIY safe
Confirm the boiler is plugged in securely and that the fuse in the spur or consumer unit has not tripped. Avoid using extension leads. If you notice flickering lights or other signs of electrical instability in your home, mention this to the engineer as it could be contributing to the fault.
- Do not remove the boiler casing or attempt to inspect internal components yourself Gas Safe engineer
The thermistor, its wiring, and connectors are located inside the boiler casing. Opening the casing as an unqualified person is unsafe and will also invalidate any remaining warranty on your Navien unit. Leave all internal inspection to a Gas Safe registered engineer.
- Have a Gas Safe engineer inspect the cold water inlet 2 thermistor and wiring Gas Safe engineer
A qualified engineer will open the casing safely, locate the thermistor, and use a multimeter to measure its resistance and voltage against the manufacturer's specification. They will also check the associated wiring harness for continuity and inspect the connector for corrosion or poor seating.
- Replace the faulty thermistor or repair the wiring as required Gas Safe engineer
If the thermistor's readings are out of specification, the engineer will fit a replacement sensor matched to your specific Navien model. If the wiring or connector is the culprit, they will repair or replace the affected section. In rare cases where the fault traces back to the PCB, the board may also need attention.
- Contact a Gas Safe registered engineer to carry out the repair Gas Safe engineer
If you have not already done so, book a Gas Safe registered engineer who has experience with Navien appliances. Check the Gas Safe Register at gassaferegister.co.uk to find an engineer in your area. If your boiler is still within its warranty period, contact Navien UK directly before arranging third-party repairs, as independent work may affect your cover.
Parts you may need
- Cold water inlet thermistor (Navien NPE/NCB compatible) · from £25
- Thermistor wiring harness · from £35
- Navien PCB (control board) · from £280
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 E432 fault code myself?
The only DIY-safe steps are resetting the boiler and checking the mains electrical supply is on and stable. Everything beyond that — inspecting the thermistor, testing resistance and voltage, and replacing parts — requires the boiler casing to be opened. This must only be done by a Gas Safe registered engineer. Attempting internal repairs yourself is unsafe and will void any remaining Navien warranty.
What is the difference between Navien fault codes E432 and E421?
Both codes relate to cold water inlet thermistor faults, but they refer to different sensors. E421 indicates a fault with the cold water inlet 1 thermistor (the primary sensor), while E432 points to the cold water inlet 2 thermistor. The diagnosis and repair process is essentially the same for both, but the engineer will need to identify and work on the correct sensor for whichever code is displayed.
How much does it cost to fix a Navien E432 fault in the UK?
For most homeowners, a thermistor replacement including call-out, labour, parts, and VAT typically costs in the region of £120 to £300. If the fault turns out to involve the PCB rather than the sensor alone, costs can rise to around £400 or more, as control boards are significantly more expensive. Getting the boiler diagnosed early usually keeps costs at the lower end, as wiring and connector faults are cheaper to fix than component replacements.
Will resetting the boiler clear the E432 fault permanently?
A reset may temporarily clear the code, but if the underlying fault — a failed thermistor, damaged wiring, or loose connector — has not been fixed, E432 will return. Repeated resets without investigation are not recommended, as the boiler is displaying E432 to protect itself. If the fault reappears after one or two resets, arrange for a Gas Safe engineer to inspect the unit.