Baxi E.04-.10 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.04-.10 fault code mean?
The Baxi E.04 series flags a permanent safety lockout linked to a problem with the gas valve or gas/air unit. The boiler shuts down and will not restart until the underlying fault is resolved. The .10 sub-code narrows the scope further: depending on your Baxi model, it can indicate a faulty external temperature probe, a gas valve wiring or connection issue, low system water pressure, or a central heating NTC sensor fault. Because the boiler has locked out for a safety-related reason, a reset alone will rarely fix it — the root cause must be identified first. Other sub-codes in the E.04 family (e.g. .11 through .17) point to related gas valve or gas/air unit faults and are handled by the same diagnostic process described on this page.
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
- Gas supply interrupted or insufficient Common
If other gas appliances such as your hob or gas fire are also not working, the supply to the property may be off or interrupted. On a prepayment meter, low credit will cut the gas feed. This is the simplest possible cause and worth ruling out before anything else.
- Frozen condensate pipe Common
In cold weather the plastic condensate pipe that runs from the boiler to an outside drain can freeze solid, preventing the boiler from firing and triggering safety-related fault codes. The pipe is usually white or grey plastic, 21–32 mm in diameter, and exits through an outside wall. This is the single most common cause of unexpected lockouts in winter months.
- Low system water pressure Common
The E.10 sub-code is associated with low water pressure on many Baxi models. If the pressure gauge on the boiler reads below 1 bar, the boiler will lock out. Pressure can drop gradually through normal use, or more sharply if there is a leak or if radiators have recently been bled.
- Faulty external temperature probe Sometimes
The .10 sub-code specifically can indicate a fault with the external frost/weather-compensation probe fitted outside the property. A damaged, corroded, or disconnected probe sends an out-of-range signal to the boiler's control board, triggering a lockout. This is a relatively quick component replacement for an engineer.
- Gas valve wiring fault or failed gas valve Sometimes
A loose or corroded wiring connection at the gas valve, or a gas valve that has failed electrically or mechanically, is a direct trigger for the E.04 fault family. The engineer will check continuity, resistance, and the valve's response to control signals before deciding whether a repair or full replacement is needed.
- Central heating NTC sensor fault Sometimes
Some Baxi models associate the E.10 sub-code with a fault on the central heating flow or return NTC thermistor. If the sensor drifts out of its expected resistance range or its wiring becomes intermittent, the control board logs the fault as a safety lockout.
- PCB fault Rare
If the control board itself has failed — through age, a power surge, or component failure — it may misread signals from the gas valve or sensors and trigger the E.04 series code. PCB faults are less common but should be considered when all other components test correctly.
How to fix it
- Check that your gas supply is working DIY safe
Try another gas appliance in the property, such as a hob burner or a gas fire. If those are also not working, contact your gas supplier. If you are on a prepayment meter, check your credit balance. Do not attempt to work on the gas meter or supply pipework yourself.
- Check the boiler's system pressure DIY safe
Look at the pressure gauge on the front of the boiler. It should sit between 1 and 1.5 bar. If it reads below 1 bar, use the filling loop (usually a silver braided hose with two valves beneath the boiler) to top it up to around 1.2–1.5 bar. Open both valves slowly until the pressure rises, then close them fully. If the pressure regularly drops, mention this to your engineer as it may indicate a slow leak.
- Thaw a frozen condensate pipe (winter only) DIY safe
Locate the condensate pipe — the white or grey plastic pipe, typically 21–32 mm wide, that exits through an outside wall and leads to a drain. If the weather is below freezing and this pipe feels solid or has visible ice, pour warm (not boiling) water gently along its length until it is clear. Boiling water can crack the plastic. Once thawed, attempt a single boiler reset.
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with an arrow or shown in your manual) for 2–3 seconds. Allow the boiler a full minute to attempt a restart. If the fault code reappears, do not reset again repeatedly — further resets will not fix an underlying component fault and can mask the problem from your engineer.
- Inspect the external temperature probe connection Gas Safe engineer
If your boiler uses a weather-compensation probe mounted outside, an engineer should check the probe's wiring for corrosion or damage and measure its resistance to confirm whether it is within specification. If faulty, the probe is replaced. This step is not homeowner-safe as it involves electrical components on the boiler circuit.
- Gas valve wiring and connection inspection Gas Safe engineer
A Gas Safe registered engineer will check the integrity of the wiring loom and connectors at the gas valve, test the valve's solenoid coils for correct resistance, and verify that the control board is sending the right command signals. A loose or corroded connector is sometimes all that is needed; a failed valve requires full replacement.
- NTC sensor testing and replacement if required Gas Safe engineer
The engineer will use a multimeter to check the resistance of the central heating NTC sensor(s) against the manufacturer's temperature-resistance table. An out-of-range reading confirms a faulty sensor, which is then replaced. Sensor replacement on Baxi boilers is generally straightforward and inexpensive.
- PCB inspection and replacement if required Gas Safe engineer
If all other components test within specification, the engineer will inspect the PCB for signs of burning, failed capacitors, or corroded tracks, and may replace it as a last resort. This is not a DIY task under any circumstances.
- Call a Gas Safe registered engineer Gas Safe engineer
If the fault returns after your DIY checks, or if you are unsure at any point, book a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk. Do not continue to reset the boiler in the hope it will clear — this can obscure important diagnostic information.
Parts you may need
- External weather compensation probe · from £35
- Central heating NTC thermistor sensor · from £25
- Gas valve (Baxi compatible) · from £120
- Printed circuit board (PCB) · 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–£320, depending on the underlying cause.
Frequently asked questions
Can I keep resetting a Baxi E.04-.10 fault to get my heating back on?
It is not advisable. A single reset is a reasonable first step, but if the fault code returns the boiler has detected a genuine problem — usually with the gas valve circuit, a sensor, or system pressure. Repeated resets can cause wear on components and will not resolve the underlying issue. Book an engineer if the fault comes back after one reset attempt.
What is the difference between the E.04 sub-codes — for example .10 versus .11 or .17?
All sub-codes in the E.04 series share the same root cause category: a fault with the gas valve or gas/air unit. The .10 sub-code specifically points toward an external probe fault, low system water pressure, or an NTC sensor issue on many Baxi models. Sub-codes .11 through .17 indicate related gas valve or gas/air unit conditions. In practice, a Gas Safe engineer will diagnose the exact component at fault rather than relying solely on the sub-code number, as the display code is a starting point rather than a precise component label.
How much does it cost to fix a Baxi E.04-.10 fault in the UK?
For the most common repairs — an NTC sensor replacement, external probe swap, or gas valve wiring fix — most homeowners pay between £100 and £320 including the callout, labour, and parts. A full gas valve replacement sits toward the upper end of that range. A PCB replacement can cost more (typically £250–£450 including labour) but is relatively rare for this fault code. Engineers in London and the South East typically charge 15–25% more than the national average. If your boiler is over 10–12 years old and repair costs approach half the price of a new boiler, it is worth getting a replacement quote at the same time.
My Baxi shows E.04-.10 only in cold weather — what does that mean?
A fault that appears exclusively in cold weather is a strong indicator of a frozen condensate pipe. The condensate pipe carries acidic water vapour away from a condensing boiler to an outside drain, and it can freeze in sub-zero temperatures. Try thawing it with warm water as described in the steps above. If the fault still returns in cold weather after the pipe is clear, an engineer should check the external weather-compensation probe, which can also behave erratically in extreme cold if its wiring or casing is damaged.