Baxi E104 Fault Code Explained

What does E104 mean on a Baxi boiler?

E104 is a safety lockout code, which means the boiler has shut itself down deliberately rather than carry on running with a fault. Exactly what it is flagging depends a little on your model range, so it is worth being honest about that.

Most UK heating sources describe E104 as a repeated loss-of-flame fault, where the boiler has lit and then lost its flame around five times within a 24-hour period and locked out to be safe.

Some references also note that on certain Baxi models E104 can indicate a faulty Safety Assist PCB — a problem with the printed circuit board that supervises the boiler's safety functions.

The practical takeaway is the same in both readings: E104 sits in the boiler's safety and combustion-control circuit. That is a sealed, gas-side area of the appliance, so it is firmly engineer territory — there is no safe DIY repair. If your display shows a slightly different number, our Baxi fault code hub covers the rest of the range.

Common causes of E104

Because E104 spans the flame circuit and, on some models, the control board, an engineer will usually be looking at one of the following:

• Repeated loss of flame — the burner igniting then going out, tripping the lockout after several attempts.
• A gas supply problem — low gas pressure, a partially closed gas valve, or a supply issue affecting other appliances too.
• A faulty flame-sensing or spark electrode — a dirty, worn or wrongly positioned electrode that cannot reliably detect the flame.
• A faulty Safety Assist PCB — on some models, the control board itself failing or losing a safe reading from its sensors.
• Wiring or connection faults — damaged cables or loose connectors between the PCB and the ignition components.
• Condensation or damp reaching the electronics, occasionally implicated in PCB faults.

What you can safely check

E104 is an engineer-only fault, so do not open the boiler or touch any gas, flue or sealed components. There are only a few genuinely safe checks a homeowner should make:

1. Check your gas is on. Make sure the gas supply isn't off and that other gas appliances (hob, fire) are working. If you have a prepayment meter, check it isn't out of credit.
2. Try one reset. Press and hold the reset button as described in your manual, then wait for the boiler to attempt to relight. Our guide on how to reset your boiler walks through this. Do this once only — repeated resets on a safety lockout can mask a real fault.
3. Glance at the pressure gauge. It should sit at roughly 1–1.5 bar when cold. If it is low, see what your boiler pressure should be and boiler pressure too low. Low pressure isn't the usual cause of E104, but it is worth ruling out.
4. In cold weather, check the condensate pipe. A frozen external condensate pipe can cause lockouts; thawing it is a recognised homeowner task. See frozen condensate pipe.

If E104 returns after a single reset, stop there. Continuing to reset a safety lockout is not a fix and can be unsafe.

When to call a Gas Safe registered engineer

If the code comes straight back, or you cannot clear it with the safe checks above, it is time to book a professional. The boundary is simple: anything involving the PCB, gas valve, burner, electrodes, flue or sealed combustion circuit must be done by a Gas Safe Register engineer. This is a legal requirement, not just best practice.

A competent engineer will typically:

• Read the fault history and live data from the boiler to confirm whether this is a flame-loss or a PCB problem.
• Check the gas supply working pressure and the gas valve.
• Inspect and, if needed, clean or reposition the ignition and flame-sensing electrodes.
• Test the wiring and connectors between the PCB and the combustion components.
• Replace the Safety Assist PCB if it is confirmed faulty. This relates to a more general boiler lockout situation, and a stubborn faulty gas valve can present similarly.

Typical Baxi E104 repair cost

Costs vary by region, boiler age and parts availability. These are indicative 2026 UK ranges; your engineer will quote after diagnosis. For wider context see our boiler repair costs guide.

Work	Indicative 2026 cost
Diagnostic / call-out	£70–£120
Flame-sensing / spark electrode replacement	£100–£220
Gas valve replacement	£180–£350
Safety Assist PCB (control board) replacement	£350–£650
Wiring / connector repair	£90–£180

Where a major part such as the PCB costs a large share of a new boiler, an engineer may discuss whether replacement makes more sense than repair on an older appliance.

Related Baxi codes

If your boiler shows a different number, these sibling guides may help:

• Baxi E110 — boiler over-temperature / overheat.
• Baxi E133 — gas supply / ignition fault (and check for a frozen condensate pipe).
• Baxi E160 — fan fault.

Will boiler cover pay for an E104 repair?

It depends on your policy — this is general information, not a guarantee about your cover.

A sudden boiler breakdown like an E104 lockout is the kind of fault many boiler-cover policies are designed for, and a plan that includes parts and labour may fund the engineer's diagnosis and a replacement part such as an electrode, gas valve or the Safety Assist PCB, subject to any excess.

Common exclusions to watch for include pre-existing faults, boilers beyond an age limit, and claims made during a policy's initial waiting period, so always check the terms of your own policy. For background see what boiler cover includes and our honest take on whether boiler cover is worth it.

This article is general information, not gas-safety or financial advice. Always have gas appliances checked and repaired by a Gas Safe registered engineer. In a gas emergency, call the National Gas Emergency line on 0800 111 999. Costs are indicative UK guides for 2026.