E18 Boiler Fault Code: What It Means on Every Brand

The E18 fault code appears on 3 boiler brands — Andrews, 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 3 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 E18 means	Severity	DIY?
AAndrews	The E18 code on Andrews boilers and water heaters covers two closely related situations depending on the model. Full guide →	Lockout — engineer likely	No
JSJohnson & Starley	E18 on a Johnson & Starley unit is the third of the stuck-sensor lockouts: a supervised temperature channel has frozen — the reading refuses to move while the unit's actual temperature… Full guide →	Lockout — engineer likely	No
KKeston	The E18 code on a Keston boiler signals that the water temperature in the flow pipe has climbed beyond safe operating limits — a condition known as flow overheat. Full guide →	Lockout — engineer likely	No

E18 brand by brand

Andrews E18

The E18 code on Andrews boilers and water heaters covers two closely related situations depending on the model. On most Andrews boilers sharing a common control platform, E18 signals a temperature sensor error — specifically, the boiler's electronics have detected an unusually rapid temperature change on the NTC (Negative Temperature Coefficient) sensor, typically exceeding 30°C in a very short period. This suggests the sensor itself is faulty or cracked, or that something is causing abnormal temperature swings in the system (such as poor water flow or an airlock). On certain Andrews commercial water heaters — notably the CWH30/200 range and models using the Andrews CWH Master Board — E18 instead points to an internal RAM fault on the control board, meaning the board's own memory or processing has encountered an error. In either case, the boiler will lock out for safety and require attention before it will fire again.

Causes, fixes & costs for Andrews E18 → · All Andrews codes

Johnson & Starley E18

E18 on a Johnson & Starley unit is the third of the stuck-sensor lockouts: a supervised temperature channel has frozen — the reading refuses to move while the unit's actual temperature changes. The controller cross-checks its sensors against expected behaviour every cycle, and a static value during heat-up fails that test immediately. As with E16 and E17 (the same supervision on the other channels), the code's job is to point the engineer at the right circuit; the underlying fault is a failed thermistor, a lead pinning the signal, or — rarely — the board input. The repair is short: confirm with a resistance check, replace the failed element, verify readings track through a full cycle.

Causes, fixes & costs for Johnson & Starley E18 → · All Johnson & Starley codes

Keston E18

The E18 code on a Keston boiler signals that the water temperature in the flow pipe has climbed beyond safe operating limits — a condition known as flow overheat. When this happens, the boiler shuts itself down and enters lockout mode as a protective measure. The root cause is almost always related to poor water circulation: if hot water cannot move freely through the system, heat builds up rapidly in the heat exchanger and triggers the overheat sensor. Note that E25 (flow temperature rising too quickly) is a closely related code and points to the same family of circulation faults. Keston boilers use a twin-flue design with a modulating pump, which means not every Gas Safe engineer will be familiar with them — it is worth specifically seeking out someone with Keston experience.

Causes, fixes & costs for Keston E18 → · All Keston codes

Frequently asked questions

Is the E18 fault code the same on every boiler?

Broadly, yes — on Andrews, Johnson & Starley and Keston the E18 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 E18 fault myself?

Not usually. On every brand listed, E18 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 E18 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.