Baxi E.04-.06 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.04-.06 fault code mean?
The E.04 fault code on Baxi boilers relates to the flue gas temperature sensor and its safety circuit. Because the boiler has permanently locked out, it will not restart on its own — a manual reset is required at minimum. The three sub-codes each point to a distinct problem: E.04-.04 and E.04-.05 indicate an electrical fault with the flue NTC thermistor, such as a short-circuit, open-circuit, or a poor wiring connection between the sensor and the PCB. E.04-.06 goes a step further and signals that the flue gas temperature actually reached a critically high level, or that circulation through the heat exchanger has failed, or that the sensor is falsely reporting an overtemperature condition. In all three cases the boiler has shut itself down as a precaution to prevent damage or danger.
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 flue NTC thermistor (sensor failure) Common
The flue NTC thermistor is a small resistance-based component whose electrical output changes in proportion to temperature. Over time the sensor can drift out of its acceptable resistance range — or fail outright with a short or open circuit — causing the PCB to read either an implausibly low or dangerously high flue temperature and triggering a permanent lockout. This is the most common root cause across all three sub-codes.
- Loose, corroded, or disconnected sensor wiring Common
The wiring loom or connector plug running from the flue sensor to the PCB can work loose over time, especially after vibration or thermal cycling. Corrosion on the connector pins also increases resistance and produces erratic readings. E.04-.04 and E.04-.05 are particularly associated with this kind of connection fault.
- Poor or failed water circulation (pump fault or airlock) Common
Relevant primarily to E.04-.06. If the central heating pump has seized, is running too slowly, or an airlock has formed in the pipework, hot water cannot be drawn away from the heat exchanger efficiently. Heat then builds up, flue gas temperatures rise to critical levels, and the boiler locks out on overtemperature protection.
- Low system pressure reducing heat dissipation Sometimes
When system pressure drops below roughly 0.5–1 bar, there is insufficient water volume to absorb and carry heat away from the heat exchanger. This can push flue gas temperatures high enough to trigger E.04-.06, even when the pump itself is working correctly.
- Partial flue blockage or restriction Sometimes
A bird's nest, debris, or a crushed terminal cap can partially obstruct the flue. Exhaust gases back up, dwell longer in the heat exchanger, and exit at a higher temperature — enough to trip the flue overtemperature threshold and produce an E.04-.06 lockout.
- PCB fault misreading sensor signal Rare
In rare cases the PCB itself develops a fault on the sensor input circuit, generating spurious readings that look like a sensor failure even when the thermistor and wiring are both sound. This is uncommon and is usually only confirmed after sensor and wiring have been ruled out.
How to fix it
- Reset the boiler once or twice DIY safe
Locate the reset button on the front panel of your Baxi boiler and press and hold it for approximately 3–5 seconds until the boiler attempts to restart. If the E.04 code returns immediately or within a short time, do not keep resetting — repeated resets without fixing the underlying fault can mask the problem and may cause further damage. Move on to the checks below.
- Check and top up system pressure if needed DIY safe
Look at the pressure gauge on the boiler fascia. When the system is cold, the needle should sit between 1 and 1.5 bar. If it reads below 1 bar, use the filling loop (usually a braided hose beneath the boiler with one or two quarter-turn valves) to slowly admit mains water until the gauge reaches around 1.2–1.5 bar, then close the valves. Do not overfill beyond 2 bar. A correctly pressurised system helps the heat exchanger stay cool and reduces the risk of an E.04-.06 overtemperature lockout.
- Bleed the radiators to clear any airlocks DIY safe
If radiators feel cold at the top or you can hear gurgling when the heating runs, trapped air may be restricting circulation. Turn the heating off and let the system cool slightly, then use a radiator bleed key on each radiator's bleed valve — starting on the ground floor and working upward — until water flows steadily without spluttering. Check the boiler pressure again afterwards and top up if it has dropped. Clearing an airlock can resolve E.04-.06 if poor circulation was the trigger.
- Check that the gas supply to the boiler is live DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If they are not, the issue may be with your gas supply rather than the boiler — contact your gas supplier or National Grid Gas Emergency on 0800 111 999 if you suspect a supply problem or smell gas.
- Inspect the flue terminal for obvious external blockages DIY safe
From outside the property, visually check the flue terminal (the white or grey cap where exhaust exits the wall or roof) for any obvious obstruction such as a bird's nest, leaves, or ice build-up. Do not attempt to dismantle the flue. If you can see a blockage that is safely reachable, remove it — but if there is any doubt about flue integrity, leave it for a Gas Safe engineer.
- Call a Gas Safe registered engineer for diagnosis and repair Gas Safe engineer
If the fault persists after the homeowner checks above, a Gas Safe registered engineer is required. They will: test the flue NTC thermistor's resistance with a multimeter and compare it against the manufacturer's specification — replacing the sensor if it is out of range; inspect the sensor wiring loom and connector for damage, corrosion, or loose pins; verify the circulation pump is running at the correct speed and flow rate, and replace it if faulty; carry out a full flue integrity and flow check; and test the PCB sensor input circuit if the sensor and wiring are both found to be in good condition. Do not attempt to work on the sensor, wiring, pump, or PCB yourself.
Parts you may need
- Flue NTC thermistor (flue gas temperature sensor) · from £55
- Sensor wiring loom / connector harness · from £30
- Central heating circulating pump · from £120
- PCB (printed circuit 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 £100–£320, depending on the underlying cause.
Frequently asked questions
What is the difference between Baxi E.04-.04, E.04-.05, and E.04-.06?
E.04-.04 and E.04-.05 are both electrical faults with the flue temperature sensor — the PCB has detected either a short-circuit or an open-circuit in the sensor or its wiring. They are treated in practically the same way: the engineer will test the sensor's resistance and inspect the wiring. E.04-.06 is different in that it signals an actual overtemperature event — the flue gas genuinely reached a critical level — or that water circulation has failed, or that the sensor is producing a false high reading. It therefore has a wider set of possible causes including a failed pump, an airlock, or low system pressure.
Can I fix a Baxi E.04 fault myself?
A homeowner can safely try a boiler reset (once or twice), top up the system pressure via the filling loop, and bleed the radiators to clear an airlock. These simple checks cost nothing and occasionally resolve E.04-.06 if low pressure or trapped air was the trigger. However, testing or replacing the flue NTC sensor, inspecting the wiring loom, servicing the circulation pump, and any work on the PCB or gas components must be carried out by a Gas Safe registered engineer. Attempting this work without the correct registration is illegal and potentially dangerous.
How much does it typically cost to fix a Baxi E.04 fault in the UK?
For most households the repair falls between £100 and £320, depending on what the engineer finds. A flue NTC sensor replacement (the most common fix) usually costs £80–£180 including labour. Replacing a circulation pump typically runs £150–£320 all in. If the PCB turns out to be at fault, the cost rises considerably — a replacement board plus fitting can reach £400–£600 — but this is a less common outcome and is worth mentioning to your engineer before authorising the work.
Why does my Baxi boiler keep locking out with E.04 even after a reset?
A permanent lockout that returns quickly after resetting almost always means the underlying fault has not been resolved. The most likely culprits are a flue NTC sensor that has failed electrically, a loose wiring connector, a pump that is struggling to circulate water, or — if you have E.04-.06 — a recurring overtemperature condition caused by persistently low system pressure or a partial flue restriction. An annual boiler service is the best way to catch these issues early before they cause a lockout mid-winter.