E34 Boiler Fault Code: What It Means on Every Brand

The E34 fault code appears on 2 boiler brands — Johnson & Starley and Keston — 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 E34 means	Severity	DIY?
JSJohnson & Starley	E34 on a Johnson & Starley unit is a low power supply lockout: the mains voltage reaching the appliance has dropped below the threshold the control board requires for reliable operation. Full guide →	Lockout — engineer likely	Some checks
KKeston	The E34 code has two distinct meanings depending on which Keston model you own — so the first step is always to confirm your exact model number. Full guide →	Lockout — engineer likely	No

E34 brand by brand

Johnson & Starley E34

E34 on a Johnson & Starley unit is a low power supply lockout: the mains voltage reaching the appliance has dropped below the threshold the control board requires for reliable operation. Gas valves, ignition circuits and safety relays all have minimum-voltage requirements — operate them undervoltage and their behaviour becomes unpredictable — so the controller shuts down rather than run marginal. The cause is usually outside the unit: a genuine supply dip in the property or wider network, a poor connection at the fused spur or in the appliance's supply wiring adding voltage drop, or, less often, a fault in the board's own power section misreading the supply.

Causes, fixes & costs for Johnson & Starley E34 → · All Johnson & Starley codes

Keston E34

The E34 code has two distinct meanings depending on which Keston model you own — so the first step is always to confirm your exact model number. On the Keston C36 Combi, E34 indicates that the mains supply voltage feeding the boiler has fallen below 180V, pointing to an electrical supply issue rather than a fault with the boiler itself. On the Keston C36P Combi and similar newer models, E34 means the boiler has experienced a flame dropout — it either failed to establish a flame during ignition or lost the flame unexpectedly during operation. In both cases the boiler will lock out and refuse to restart until the fault is addressed and the reset button is pressed.

Causes, fixes & costs for Keston E34 → · All Keston codes

Frequently asked questions

Is the E34 fault code the same on every boiler?

Broadly, yes — on Johnson & Starley and Keston the E34 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 E34 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.

Who should I call about a E34 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.