E33 Boiler Fault Code: What It Means on Every Brand

The E33 fault code appears on 3 brands — Andrews, Johnson & Starley and Keston — 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 E33 means	Severity	DIY?
AAndrews	The E33 code on Andrews water heaters and boilers indicates that the control board has detected a problem with the NTC (Negative Temperature Coefficient) thermistor fitted to the central… Full guide →	Lockout — engineer likely	No
JSJohnson & Starley	E33 on a Johnson & Starley unit reports a return thermistor fault: the sensor measuring return temperature — return air on warm-air models, heating return water on the combined and… Full guide →	Engineer may be needed	No
KKeston	The E33 code points to a problem with the domestic hot water (DHW) thermistor circuit — the sensor responsible for monitoring the temperature of your hot water. Full guide →	Lockout — engineer likely	No

E33 brand by brand

Andrews E33

The E33 code on Andrews water heaters and boilers indicates that the control board has detected a problem with the NTC (Negative Temperature Coefficient) thermistor fitted to the central heating return pipe — the pipe carrying cooler water back into the appliance. The sensor has either lost its electrical connection completely (open circuit) or developed an internal short circuit. Either condition means the PCB can no longer receive reliable temperature data from the return side, so the appliance locks out to protect itself. Andrews systems share a European control architecture with other appliances in the same manufacturing group, and E33 is consistently used across that platform to flag this specific return sensor fault. Note: on a small number of other boiler platforms the E33 code can indicate a flame-detection issue rather than a sensor fault — always cross-check against your specific Andrews model's installation manual if you are unsure.

Causes, fixes & costs for Andrews E33 → · All Andrews codes

Johnson & Starley E33

E33 on a Johnson & Starley unit reports a return thermistor fault: the sensor measuring return temperature — return air on warm-air models, heating return water on the combined and water-side variants — has failed electrically, reading open or short circuit rather than a plausible temperature. The controller uses the return reading to judge how much heat the house is absorbing and to modulate output accordingly, so it restricts operation when that probe fails. Diagnosis is the classic thermistor routine — inspect the wiring, measure resistance against the temperature table, replace the failed part — and it sits in the same family as codes 43 and 44, which cover specific return-sensor failure modes.

Causes, fixes & costs for Johnson & Starley E33 → · All Johnson & Starley codes

Keston E33

The E33 code points to a problem with the domestic hot water (DHW) thermistor circuit — the sensor responsible for monitoring the temperature of your hot water. The precise nature of the fault differs slightly between models: on the Keston C36 Combi, E33 indicates an open circuit (a broken or disconnected connection to the DHW sensor), whereas on the Keston Qudos 28s, E33 signals a closed or short circuit (wires that are touching or bridged when they shouldn't be). In both cases, the boiler's control board is receiving an abnormal signal — or no signal at all — from the DHW thermistor, so it shuts down hot water production as a precaution. The boiler will not resume normal operation until the fault is cleared and the reset button is pressed; simply switching the boiler off and back on will not clear it.

Causes, fixes & costs for Keston E33 → · All Keston codes

Frequently asked questions

Is the E33 fault code the same on every boiler?

No. E33 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 E33 fault myself?

Not usually. On every brand listed, E33 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 E33 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.