E35 Boiler Fault Code: What It Means on Every Brand

The E35 fault code appears on 3 brands — Johnson & Starley, Keston and Warmflow — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Lockout — engineer likely 3 brands use this code Meaning differs by brand

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 E35 means	Severity	DIY?
JSJohnson & Starley	E35 on a Johnson & Starley unit reports a flow thermistor fault: the sensor measuring the flow-side temperature — the heated air leaving the unit on warm-air models, or the heating flow… Full guide →	Engineer may be needed	No
KKeston	The E35 code appears when your Keston boiler detects a problem with the incoming electrical supply and locks out as a safety measure. Full guide →	Lockout — engineer likely	No
WWarmflow	The E35 fault code on a Warmflow boiler indicates that the control board has lost confidence in the ionisation (flame-sensing) electrode. Full guide →	Lockout — engineer likely	No

E35 brand by brand

Johnson & Starley E35

E35 on a Johnson & Starley unit reports a flow thermistor fault: the sensor measuring the flow-side temperature — the heated air leaving the unit on warm-air models, or the heating flow water on combined and water-side variants — has failed electrically, reading open or short. The flow reading is the controller's primary handle on output temperature: it modulates the burner against it and backs the overheat protection with it, so a failed flow probe restricts operation immediately. It is the counterpart of E33 (return thermistor); together the pair give the controller the temperature rise across the heat exchanger. Diagnosis and repair are the standard thermistor routine — wiring inspection, resistance check against the table, replacement.

Causes, fixes & costs for Johnson & Starley E35 → · All Johnson & Starley codes

Keston E35

The E35 code appears when your Keston boiler detects a problem with the incoming electrical supply and locks out as a safety measure. The exact definition varies slightly by model: on the C36 Combi and C36P Combi, E35 means the mains voltage has dropped below 180V; on older Keston twin-flue models, it indicates the mains frequency is outside the expected 50Hz tolerance. In both cases the boiler has sensed something abnormal about the electricity reaching it — either the voltage is too low or the frequency is off — and has stopped operating until the issue is resolved. This is an electrical fault rather than a gas or heating-component fault, which means a qualified electrician may need to be involved alongside a Gas Safe engineer.

Causes, fixes & costs for Keston E35 → · All Keston codes

Warmflow E35

The E35 fault code on a Warmflow boiler indicates that the control board has lost confidence in the ionisation (flame-sensing) electrode. During every ignition attempt, the PCB passes a tiny electrical current through the flame itself — if a signal returns, the boiler knows combustion is happening safely and continues to supply gas. If that signal fails to come back within a few seconds of the spark firing, the boiler cuts the gas supply, locks out, and displays E35 as a safety precaution. This prevents unburned gas from building up inside or around the appliance. The root cause is most often a dirty or worn electrode, but wiring faults, combustion issues, or a failing PCB can also trigger this code.

Causes, fixes & costs for Warmflow E35 → · All Warmflow codes

Frequently asked questions

Is the E35 fault code the same on every boiler?

No. E35 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a E35 fault myself?

Not usually. On every brand listed, E35 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 E35 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.