Baxi E.01-.04 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.01-.04 fault code mean?
The Baxi E.01–E.04 family covers a range of locking faults — meaning the boiler shuts down completely and will not restart until the underlying problem has been fixed and the appliance reset. Each series targets a specific fault area: E.01 covers flame loss and temperature anomalies (e.g. E.01.02 — return sensor reading higher than flow sensor; E.01.04 — flame lost five times in 24 hours; E.01.11 — incorrect fan speed; E.01.20 — maximum flue gas temperature reached). E.02 covers system and communication failures (e.g. E.02.07 — water pressure below 0.5 bar; E.02.16 — CSU time-out or PCB damage; E.02.17 — loss of communication between the gas valve and PCB). E.03 points to faults with the draught sensor, flue thermostat, or pneumatic relay. E.04 covers permanent temperature sensor and ignition lockouts (e.g. E.04.01 — flow sensor short-circuit; E.04.02 — flow sensor open-circuit; E.04.03 — critical flow temperature with poor circulation; E.04.08 — safety thermostat open-circuited; E.04.10 / E.04.11 — failure to ignite after four attempts; E.04.13 — fan impeller blocked or overspeed; E.04.14 — gas valve control circuit fault). When you see a full code such as E.01.04 or E.04.11 on your display, find the sub-code in the descriptions above to understand exactly what your boiler is reporting.
General guidance only — not a substitute for professional advice. 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 ignition or flame-sensing electrode Common
Over time the ceramic insulator on the electrode can crack, or combustion deposits build up on the tip, preventing a reliable spark or stopping the PCB from receiving a flame signal. The boiler may light momentarily but the PCB sees no confirmation and locks out — the classic pattern behind E.01.04, E.04.10, and E.04.11.
- Gas supply interruption or low gas pressure Common
If the gas meter is off, the gas pressure at the appliance is below the required level, or a pre-payment meter has run out, the burner cannot establish a flame. Check whether other gas appliances in the property are working — if they are not, the fault lies upstream of the boiler.
- Low or lost system water pressure (E.02.07) Common
When system pressure drops below approximately 0.5 bar — usually due to a slow leak or air in the system — the boiler refuses to fire. This is one of the few E-series faults a homeowner can address directly by topping up via the filling loop.
- Flow or return temperature sensor failure Common
A short-circuited sensor (E.04.01), open-circuited sensor (E.04.02), or a sensor reporting temperatures in the wrong order (E.01.02, E.01.12) will trigger an immediate lockout. Sensors degrade with age and can also fail due to poor wiring connections.
- Fan or air pressure switch fault Sometimes
The fan must reach the correct speed before ignition is permitted. A worn fan motor, blocked impeller (E.04.13), or a failed air pressure switch means the airflow condition is never satisfied and the boiler locks out before the burner even attempts to light (also linked to E.01.11).
- Faulty or blocked gas valve Sometimes
A gas valve that opens too slowly, partially, or not at all delivers insufficient gas to the burner during the ignition window. The spark fires but the flame cannot establish, leading to repeated lockouts. E.04.14 points specifically to a fault in the gas valve control circuit.
- PCB (printed circuit board) failure Sometimes
The PCB orchestrates ignition timing, flame detection, fan control, and sensor monitoring. A failing PCB can produce almost any E-code, often intermittently or without an obvious pattern. Suspected after other components have been checked and ruled out.
- Frozen or blocked condensate pipe Sometimes
In cold weather the condensate pipe — which carries acidic waste water from the boiler to the drain — can freeze solid. This prevents the boiler from operating safely and may trigger lockout codes, particularly on condensing models.
- Draught sensor or flue thermostat fault (E.03) Rare
The draught sensor monitors flue gas flow; if it detects a blockage, downdraught, or simply fails electrically, the boiler locks out to prevent harmful combustion gases entering the home. A failed pneumatic relay can produce the same result.
- Safety thermostat open-circuit (E.04.08) Rare
The high-limit safety thermostat trips when the heat exchanger reaches a dangerously high temperature — usually because circulation has been lost or a pump has failed. Once tripped it must be investigated and reset by an engineer; it will not restore itself.
How to fix it
- Check that the gas supply is active DIY safe
Turn on a gas hob or gas fire elsewhere in the property. If these also fail to light or ignite poorly, the issue is with the gas supply to the building — contact your gas network operator (e.g. Cadent, SGN, Wales & West) rather than a boiler engineer.
- Check and top up system water pressure if the display shows E.02.07 DIY safe
Look at the pressure gauge on the boiler — it should normally sit between 1.0 and 1.5 bar when the system is cold. If the needle is at or below 0.5 bar, use the filling loop (a flexible braided hose or a built-in valve, usually beneath the boiler) to slowly add water until the gauge reads around 1.2–1.3 bar, then close the valve. Consult your boiler manual if you are unsure where the filling loop is located.
- Check for a frozen condensate pipe in cold weather DIY safe
If the outside temperature is near or below freezing, locate the white plastic condensate pipe that exits the boiler and runs to an external drain or soakaway. If it feels solid or no water drips from the end, it is likely frozen. Pour warm (not boiling) water along its length or wrap it in a warm, damp cloth until it thaws, then proceed to reset the boiler.
- Reset the boiler — no more than two or three times DIY safe
Press and hold the reset button (usually marked with a flame or the word RESET on the front panel) for 3–5 seconds, then release. Allow the boiler up to two minutes to complete its start-up sequence. If the same fault code returns after two or three resets, stop resetting — repeated attempts will not fix a component fault and may mask important diagnostic information for your engineer.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
All work beyond the checks above — including inspecting or replacing the ignition electrode, ionisation probe, gas valve, PCB, temperature sensors, fan, air pressure switch, draught sensor, or safety thermostat — must be carried out by a Gas Safe registered engineer. Ask for their Gas Safe registration number before work begins; you can verify it at gassaferegister.co.uk. If your boiler is still under its Baxi warranty (up to 10 years with an accredited installer and annual services maintained), contact Baxi directly before commissioning independent repairs.
Parts you may need
- Ignition / ionisation electrode · from £25
- Gas valve · from £120
- Flow temperature sensor (NTC thermistor) · from £18
- Flue / return temperature sensor · from £18
- Fan assembly · from £95
- Air pressure switch · from £30
- PCB (printed circuit board) · from £180
- Safety thermostat (high-limit stat) · from £20
- Draught sensor / pneumatic relay · from £35
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–£380, depending on the underlying cause.
Frequently asked questions
What is the difference between E.01, E.02, E.03, and E.04 on a Baxi boiler?
All four are locking fault series, meaning the boiler shuts down completely and needs both a repair and a manual reset to restart. E.01 deals with flame loss and temperature sensor conflicts — for example, E.01.04 means the flame has been lost five times in 24 hours. E.02 covers system-level issues such as low water pressure (E.02.07) and communication failures between the PCB and gas valve (E.02.17). E.03 points to a problem with the draught sensor, flue thermostat, or pneumatic relay. E.04 is the broadest series and covers permanent ignition failures (E.04.10, E.04.11), temperature sensor short and open circuits (E.04.01, E.04.02), fan faults (E.04.13), and gas valve control circuit failures (E.04.14).
Can I fix a Baxi E.01 or E.04 fault myself?
A few preliminary checks are safe to do yourself: confirming the gas supply is on, topping up system pressure via the filling loop if the code is E.02.07, thawing a frozen condensate pipe in cold weather, and attempting a reset two or three times. Beyond that, the fault involves gas components, electrical control boards, or safety devices that must only be worked on by a Gas Safe registered engineer. Attempting DIY repairs on these parts is illegal for unregistered individuals and potentially dangerous.
How much does it typically cost to fix a Baxi E.01–E.04 fault code in the UK?
Most repairs fall in the region of £120–£380 all-in, depending on which component has failed. An electrode or sensor replacement tends to be at the lower end (roughly £100–£200 including labour), while a gas valve replacement is usually £200–£400. PCB replacement is generally £200–£450, though on older Baxi models where parts are harder to source the cost can exceed £500. Always ask for a fixed quote before work starts.
My Baxi keeps showing E.01.04 — why does the flame keep going out?
E.01.04 means the boiler has detected flame loss five times within a 24-hour period and has locked out for safety. The most common culprits are a worn or dirty ionisation electrode (which senses that a flame is present), insufficient gas pressure at the valve, a partially blocked or slow-opening gas valve, or a fan that is not providing the correct airflow for stable combustion. A Gas Safe engineer will work through the ignition and combustion sequence to identify exactly which component is failing.
Is my Baxi boiler still under warranty if it shows an E-code?
Baxi offers a minimum two-year warranty on new boilers, extendable to up to ten years when installed by a Baxi-accredited installer and serviced annually. If your boiler is within the warranty period and the services are up to date, contact Baxi's customer support line before arranging an independent engineer — warranty repairs are carried out at no parts cost to you. Note that some faults caused by limescale, incorrect installation, or failure to service annually may not be covered.