Boiler
E4
The E4 error code on a Baxi boiler indicates a fault with the ignition or flame detection system — the boiler is failing to light or cannot confirm a stable flame. This is a moderately serious fault that will lock out your boiler and leave you without heating or hot water until it is resolved.
What does this error mean?
When your Baxi boiler displays E4, it has attempted to ignite and either failed to detect a flame at all, or detected one that went out almost immediately — a condition known as ‘lockout’. The boiler’s control board triggers this code as a safety measure to prevent unburned gas from building up inside the heat exchanger. Until the underlying cause is identified and fixed, the boiler will not attempt to fire again automatically.
Est. repair cost
£80–£250 for a Gas Safe engineer repair (parts and labour); no safe DIY repair option for this fault
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: confirm other gas appliances (hob, oven) are working. If not, contact your gas supplier immediately — do not attempt to reset the boiler.
Check your prepayment meter (if applicable) has credit and top it up if needed, then wait 5 minutes before proceeding.
Inspect the condensate pipe (the white plastic pipe that exits externally, usually at low level) — if it is frozen, thaw it carefully with warm (not boiling) water and a cloth. Baxi’s own guidance supports this as a homeowner task.
Perform a boiler reset: locate the reset button (on most Baxi models this is a dedicated button or a turn-and-hold on the control dial), hold for 3–5 seconds, and wait up to 2 minutes for the boiler to attempt re-ignition.
If the E4 code returns after one or two reset attempts, stop resetting. Repeated failed ignition attempts can cause gas to accumulate — this is the point to call a professional.
Do NOT attempt to inspect, clean, or replace the ignition electrodes, flame sensor, gas valve, or PCB yourself. These tasks require a Gas Safe registered engineer by law.
📞 When to call a professional
If the boiler locks out again within a short period after a reset, or if the E4 code appears immediately on restart, call a Gas Safe registered engineer — do not keep resetting the boiler. Tell the engineer you have a Baxi boiler showing an E4 fault code, that you have already checked the gas supply and condensate pipe, and approximately how many times it has locked out. This will help them diagnose quickly and arrive with the most likely replacement parts.
⚖️ Repair or replace?
A Gas Safe engineer call-out and repair for an E4 fault typically costs £80–£250 depending on the part required — ignition electrodes and flame sensors are at the lower end, a gas valve or PCB at the upper end. If your Baxi boiler is over 10–12 years old and this is not its first significant fault, replacement becomes worth considering: a new A-rated combi boiler installed costs £1,800–£3,000 in the UK but will be more efficient and carry a fresh warranty. For a boiler under 8 years old in otherwise good condition, repair is almost always the right call.