E05 Boiler Fault Code: What It Means on Every Brand

The E05 fault code appears on 5 boiler brands — Biasi, Johnson & Starley, Keston, Ravenheat and Warmflow — 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 5 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 E05 means	Severity	DIY?
BBiasi	The E05 (also displayed as ER05 on some Biasi models) fault code indicates that the boiler has locked out due to a problem with the fan or the draught pressure circuit. Full guide →	Lockout — engineer likely	No
JSJohnson & Starley	The E05 code appears on Johnson & Starley boilers — including the Quantec, Reno HE, and WarmCair ranges — when the PCB detects a problem with the flue fan. Full guide →	Lockout — engineer likely	No
KKeston	The E05 code is a lockout fault on Keston boilers, but its precise meaning varies by model. Full guide →	Lockout — engineer likely	No
RRavenheat	The E05 fault code on Ravenheat boilers signals a problem with the boiler's flue fan. Full guide →	Lockout — engineer likely	No
WWarmflow	The E05 code on a Warmflow boiler signals a problem with the boiler flow NTC (Negative Temperature Coefficient) thermistor — the sensor responsible for monitoring the temperature of water… Full guide →	Lockout — engineer likely	No

E05 brand by brand

Biasi E05

The E05 (also displayed as ER05 on some Biasi models) fault code indicates that the boiler has locked out due to a problem with the fan or the draught pressure circuit. When your Biasi boiler starts up, the PCB (printed circuit board) checks that the fan is spinning correctly and generating enough airflow before it allows ignition. It does this by monitoring the air pressure switch, which should detect a specific pressure change as the fan ramps up. If that pressure change is absent, insufficient, or irregular, the PCB treats this as unsafe — waste combustion gases could otherwise build up inside the flue — and immediately locks the boiler out. The E05 code is the boiler's way of telling you it could not confirm safe draught conditions at start-up.

Causes, fixes & costs for Biasi E05 → · All Biasi codes

Johnson & Starley E05

The E05 code appears on Johnson & Starley boilers — including the Quantec, Reno HE, and WarmCair ranges — when the PCB detects a problem with the flue fan. This means the fan has either failed to start, is running well below its required speed, or has stopped mid-cycle. Because the fan is responsible for expelling harmful combustion gases safely through the flue, the boiler will shut itself down and lock out rather than risk those gases remaining inside the appliance or your home. A reset may clear the lockout temporarily, but if the underlying fan fault remains, E05 will return. Diagnosis and repair must be carried out by a Gas Safe registered engineer.

Causes, fixes & costs for Johnson & Starley E05 → · All Johnson & Starley codes

Keston E05

The E05 code is a lockout fault on Keston boilers, but its precise meaning varies by model. On the Keston C36 combi, E05 is most commonly associated with a fan fault — the boiler's printed circuit board cannot confirm that the fan is operating correctly, so it shuts down to prevent dangerous flue gases from building up. On some C36 variants, however, E05 is recorded as a boiler overheat fault, where an internal safety thermostat has tripped because the boiler reached an unsafe temperature. Both interpretations are credible and supported by specialist sources, so both are addressed in this guide. In either case, the boiler will lock out and refuse to restart until the reset button is pressed.

Causes, fixes & costs for Keston E05 → · All Keston codes

Ravenheat E05

The E05 fault code on Ravenheat boilers signals a problem with the boiler's flue fan. The fan is responsible for expelling combustion gases safely out through the flue, and the boiler's control board continuously monitors it to confirm it is running correctly before and during a heating or hot water demand. If the PCB cannot verify that the fan is operating as expected — whether because the fan itself has failed, the wiring is damaged, or the board is misreading the signals — it will lock the boiler out entirely as a safety precaution. This means you will have no heating and no hot water until the underlying fault is diagnosed and resolved. The E05 code appears across Ravenheat's CS, CSI, WH, and HE series boilers.

Causes, fixes & costs for Ravenheat E05 → · All Ravenheat codes

Warmflow E05

The E05 code on a Warmflow boiler signals a problem with the boiler flow NTC (Negative Temperature Coefficient) thermistor — the sensor responsible for monitoring the temperature of water circulating through the heating circuit. On the Agentis Series this is described as a Boiler NTC Error, while on the G-Series (such as the GS25A) it may appear labelled as a Heating Sensor Fault, but both refer to the same underlying issue. An NTC thermistor works by changing its electrical resistance as temperature rises or falls; the boiler's PCB reads that resistance to determine how hot the water is and adjusts the burner accordingly. When the PCB receives a signal that is absent, wildly out of range, or simply implausible, it treats the situation as potentially unsafe and locks the boiler out, displaying E05. The fault can stem from the sensor itself failing (either going open-circuit or drifting out of calibration), from wiring faults between the sensor and the PCB, or — less commonly — from a problem with the PCB itself.

Causes, fixes & costs for Warmflow E05 → · All Warmflow codes

Frequently asked questions

Is the E05 fault code the same on every boiler?

Broadly, yes — on Biasi, Johnson & Starley, Keston, Ravenheat and Warmflow the E05 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 E05 fault myself?

Not usually. On every brand listed, E05 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 E05 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.