Baxi E4 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E4 fault code mean?
The Baxi E4 code means the boiler tried to light the burner but could not confirm a stable flame within its permitted attempts. The control board detects no flame signal during the ignition sequence and locks the boiler out as a safety measure. E4 appears across several Baxi ranges — including the Baxi 200, 400, and older Platinum/Solo models — and always relates to this same ignition or flame-establishment failure. Until the underlying cause is fixed, the boiler will continue to lock out after each reset attempt.
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
- Gas supply interrupted or insufficient Common
If the gas meter is off, a prepayment meter has run out of credit, or there is a local supply interruption, the boiler cannot ignite because no gas reaches the burner. This is the first thing to rule out because it costs nothing to check.
- Faulty or contaminated ignition electrodes Common
The ignition electrodes produce the high-voltage spark that lights the burner. Prolonged exposure to heat and combustion residue causes them to become carbon-coated or physically worn, producing a weak or absent spark. Dirty electrodes can sometimes be cleaned; physically cracked or eroded ones need replacing.
- Flame sensor (ionisation probe) fault Common
Once the burner lights, a separate ionisation probe checks that a flame is actually present. If this probe is dirty, corroded, or its signal wire has a poor connection, the boiler receives no confirmation that ignition succeeded and shuts down — even if a flame did briefly appear.
- Air trapped in the heating system Sometimes
Significant air pockets in the system can cause low or erratic water flow past the heat exchanger, leading to overheat trips or pressure-related shutdowns that the boiler interprets during the ignition cycle. Bleeding radiators and checking system pressure (should be 1–1.5 bar) addresses this.
- Gas valve not opening correctly Sometimes
The gas valve controls the flow of gas to the burner. If it is sticking, worn, or has a failed solenoid coil, it may not open fully — or at all — when the PCB requests ignition. This is a Gas Safe engineer job exclusively.
- PCB (printed circuit board) fault Rare
The PCB orchestrates the entire ignition sequence. A failing PCB may send incorrect signals to the electrodes or gas valve, causing repeated ignition failure even when all other components are in good condition. PCB faults are relatively rare but more likely on older boilers.
How to fix it
- Check that your gas supply is working DIY safe
Turn on a gas hob or gas fire to confirm gas is reaching your home. If other gas appliances also fail to work, contact your gas supplier — the fault is not with the boiler itself. If you have a prepayment meter, make sure it has sufficient credit.
- Check and restore boiler system pressure DIY safe
Look at the pressure gauge on the boiler's front panel. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1 bar, use the filling loop (usually a braided hose beneath the boiler) to top up slowly until the gauge reaches 1.2–1.5 bar, then close the filling loop valves fully.
- Bleed your radiators to remove trapped air DIY safe
Switch the heating off and let the system cool. Use a radiator bleed key on each radiator's bleed valve (usually a square fitting at the top corner) until water — not air — trickles out. Re-check boiler pressure afterwards and top up again if it has dropped below 1 bar.
- Reset the boiler DIY safe
On most Baxi models, press and hold the reset button (often marked with a flame or reset symbol on the front panel) for approximately two to three seconds until the boiler attempts to restart. Allow a full ignition attempt before declaring it failed. Repeat up to two or three times if the boiler locks out again — but do not keep resetting indefinitely, as this can mask a worsening fault.
- If the fault persists, call a Gas Safe registered engineer Gas Safe engineer
Continued E4 lockouts after the checks above point to a component fault — most likely the ignition electrodes, flame sensor, or gas valve. A Gas Safe engineer will test spark output, inspect electrode condition, check the ionisation signal, and verify gas valve operation. They can clean or replace the electrodes and flame sensor, and replace the gas valve or PCB if required. Do not attempt to open the boiler casing or touch gas components yourself.
Parts you may need
- Ignition electrode (single) · from £25
- Ignition electrode pair · from £45
- Flame sensor / ionisation probe · from £20
- Gas valve · from £120
- Ignition lead set · from £18
- PCB (main 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 £150–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Baxi E4 fault myself?
The safe DIY steps are limited to confirming your gas supply is on, topping up system pressure via the filling loop, bleeding your radiators, and resetting the boiler up to two or three times. If those actions do not clear the fault, the cause is almost certainly a component inside the boiler — ignition electrodes, the flame sensor, or the gas valve — which legally requires a Gas Safe registered engineer to diagnose and repair.
Why does my Baxi boiler keep coming back with E4 after I reset it?
A single reset can clear a one-off nuisance lockout (for example, a momentary gas pressure dip), but if E4 returns every time you reset, the boiler is telling you something is consistently wrong. The most common culprits are worn or dirty ignition electrodes that can no longer produce a reliable spark, or a flame sensor that is too contaminated to confirm ignition. An engineer can usually resolve either issue in a single visit.
How much does it cost to fix a Baxi E4 fault in the UK?
Most homeowners pay between £150 and £350 for an E4 repair. Cleaning or replacing ignition electrodes and the flame sensor typically sits at the lower end — roughly £150–£220 including labour. A gas valve replacement usually falls in the £200–£350 range once parts and labour are counted. PCB replacement is less commonly required but can push costs higher; if your engineer suspects the PCB is at fault, it is worth getting a written quote and comparing it against the cost of a new boiler, particularly if your Baxi is more than 10 years old.
Does the Baxi warranty cover the E4 fault code?
It can do. New Baxi boilers carry at least a two-year warranty, and boilers installed by a Baxi-accredited installer can receive up to 10 years' cover — provided annual services have been carried out and recorded. If your boiler is still within its warranty period and has been serviced every year, contact Baxi directly before booking an independent engineer, as the repair may be covered at no cost to you.