E.04-.05 Boiler Fault Code: What It Means on Every Brand

The E.04-.05 fault code appears on 2 boiler brands — Baxi and Remeha — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 2 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What E.04-.05 means	Severity	DIY?
BBaxi	The Baxi E.04-.04 and E.04-.05 fault codes both indicate a permanent lockout caused by a problem with the flue gas temperature sensor (an NTC thermistor). Full guide →	Lockout — engineer likely	No
RRemehashown as E.04.05	The E.04.05 code belongs to Remeha's E4 fault family, which covers ignition and flame establishment failures. Full guide →	Lockout — engineer likely	No

E.04-.05 brand by brand

Baxi E.04-.05

The Baxi E.04-.04 and E.04-.05 fault codes both indicate a permanent lockout caused by a problem with the flue gas temperature sensor (an NTC thermistor). Specifically, the boiler's PCB has detected that this sensor has short-circuited, is not connected at all, or has a poor connection that is producing unreliable readings. Because the PCB continuously monitors the resistance signal from this thermistor to track flue gas temperature, a short-circuit causes wildly incorrect data to reach the control board — and as a safety precaution, the boiler shuts down completely and will not restart until the fault is resolved and a manual reset is performed. These are classed as permanent faults, meaning a reset alone will not clear them without the underlying issue being fixed first. Two related sub-codes worth knowing: E.04-.06 triggers when a critically high flue gas temperature is actually reached (often alongside a circulation or sensor fault), and E.04-.08 indicates the boiler has hit its maximum safe temperature limit, which can also involve the safety thermostat opening or a circulation failure.

Causes, fixes & costs for Baxi E.04-.05 → · All Baxi codes

Remeha E.04.05

The E.04.05 code belongs to Remeha's E4 fault family, which covers ignition and flame establishment failures. In plain terms, the boiler has attempted to light several times in succession — more than five start attempts — without successfully detecting a stable flame, so it has locked itself out as a safety measure. The sub-code .05 identifies the precise failure mode within that ignition category, pointing toward a problem with either the spark generation, the flame signal (ionisation current), or the gas reaching the burner at the right pressure. Until the underlying fault is resolved and the boiler is reset, it will not attempt to fire again. This is a deliberate safety lockout, not a random glitch.

Causes, fixes & costs for Remeha E.04.05 → · All Remeha codes

Frequently asked questions

Is the E.04-.05 fault code the same on every boiler?

Broadly, yes — on Baxi and Remeha the E.04-.05 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a E.04-.05 fault myself?

Not usually. On every brand listed, E.04-.05 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a E.04-.05 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.