Baxi E321 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E321 fault code mean?
The E321 code appears on Baxi boilers when the control board detects a problem with the hot water NTC (Negative Temperature Coefficient) thermistor — the sensor responsible for continuously measuring the temperature of the domestic hot water. Without a reliable reading from this sensor, the boiler cannot regulate hot water output safely, so it shuts down and locks out until the fault is properly resolved. In plain terms, the boiler's internal thermometer for hot water has either failed, is sending implausible readings, or has a wiring problem between the sensor and the PCB.
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 DHW NTC thermistor Common
The sensor itself has worn out or developed an internal fault, causing it to send temperature readings that are out of range or wildly inconsistent. This is the most common root cause of E321 and almost always means the sensor needs to be replaced. Signs leading up to failure often include fluctuating hot water temperatures or the boiler cutting out mid-demand.
- Wiring fault between sensor and PCB Sometimes
The low-voltage wiring loom connecting the hot water NTC sensor to the printed circuit board can suffer from loose terminals, corrosion at connectors, or chafed insulation. A poor connection produces the same erratic signal as a faulty sensor itself, so engineers will typically check wiring continuity before condemning the sensor.
- Short circuit within the sensor circuit Sometimes
A short to earth or between wires in the sensor circuit causes the PCB to read an impossibly low resistance, which it interprets as an extreme temperature — triggering the lockout. Visible damage to the wiring or sensor body is sometimes present, but internal short circuits can look normal to the naked eye.
- PCB fault misreading sensor data Rare
In a small number of cases the sensor and its wiring are electrically sound, but the PCB input channel that reads the signal has failed. This is diagnosed by substitution — fitting a known-good sensor and finding the fault persists — and is the least common cause of E321.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with a flame or 'R' symbol) for around three seconds. If the boiler fires up and runs normally, monitor it over the next hour. If E321 returns, do not reset again — repeated resets without resolving the underlying cause can mask a developing fault and will not fix a failed sensor.
- Check system pressure DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, top it up via the filling loop — consult your boiler's user manual for the correct procedure for your model. Low pressure alone will not cause E321, but it is worth ruling out before calling an engineer.
- Have a Gas Safe engineer test the NTC sensor resistance Gas Safe engineer
An engineer will disconnect the sensor and measure its electrical resistance with a multimeter at a known temperature. NTC thermistors follow a predictable resistance-temperature curve; a reading that falls well outside the expected range confirms sensor failure. This is the key diagnostic step and must be performed by a qualified professional.
- Inspect and test sensor wiring and connectors Gas Safe engineer
The engineer will check all connectors in the sensor circuit for corrosion, looseness, or damage, and perform a continuity test along the wiring loom. Any compromised wiring will be repaired or replaced. This step is carried out before or alongside sensor testing to avoid unnecessary parts replacement.
- Replace the hot water NTC thermistor if faulty Gas Safe engineer
If the sensor is confirmed defective, the engineer will fit a replacement OEM or approved equivalent part. The boiler will then be restarted, the fault code cleared, and the hot water circuit tested under normal demand conditions to confirm the repair has resolved the issue.
- Test or replace the PCB if sensor and wiring are both sound Gas Safe engineer
Should the fault persist after a confirmed-good sensor and clean wiring, the PCB input circuit is likely at fault. PCB replacement is a more involved and costly repair; the engineer may recommend a like-for-like board replacement or discuss whether the boiler's age makes a repair economically sensible.
- Call a Gas Safe registered engineer if you have not already Gas Safe engineer
If a single reset has not cleared E321, you will need a Gas Safe registered engineer to diagnose and fix the fault. You can verify any engineer's registration at www.gassaferegister.co.uk. Do not attempt to bypass the lockout or interfere with the sensor wiring yourself — this is both unsafe and likely to invalidate any remaining warranty.
Parts you may need
- Baxi DHW NTC thermistor sensor · from £18
- Sensor wiring loom / harness · from £25
- Baxi PCB (printed circuit board) · from £180
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 £100–£280, depending on the underlying cause.
Frequently asked questions
Can I fix Baxi E321 myself without an engineer?
The one DIY step worth trying is a single boiler reset. If E321 disappears and the boiler runs normally, keep an eye on it. If the fault returns, stop resetting — you need a Gas Safe registered engineer. Testing and replacing an NTC sensor involves working with electrical components inside the boiler, which should only be done by a qualified professional. Attempting it yourself risks injury, further damage, and invalidating your boiler warranty.
How much does it cost to fix a Baxi E321 fault in the UK?
For most households, the total bill — including a callout or diagnostic charge, labour, and the cost of a replacement NTC sensor — falls somewhere between £100 and £280. The sensor itself is an inexpensive part (typically under £20), so the majority of the cost is labour and the engineer's callout fee. Prices in London and the South East tend to run 15–25% higher than the national average. If the PCB is also found to be faulty, costs can rise significantly — a replacement board for a Baxi boiler typically costs £150–£200 for the part alone.
Why does the boiler lock out completely for a sensor fault?
When the hot water NTC sensor fails or sends readings that fall outside safe operating limits, the boiler's control board has no reliable way to know whether the water is dangerously overheating or barely warm. Rather than risk overheating or scalding water reaching taps, the boiler locks out entirely as a built-in safety measure. This is the correct response — it protects both the appliance and the occupants.
Will an annual boiler service help prevent E321?
Yes. During a Gas Safe engineer's annual service, sensor connections are inspected and cleaned, wiring is checked for deterioration, and any early signs of component wear are flagged before they cause a lockout. Most boiler manufacturers also require annual servicing to keep the warranty valid. A service typically costs £60–£120 and is the single most effective way to avoid unexpected breakdown costs.