E3 Boiler Fault Code: What It Means on Every Brand

E3 has six meanings across six brands — a PCB fault on Baxi, circulation on Alpha, flue thermistor on Firebird, hot-water sensor on Protherm and Remeha's own sensor fault, and a burner lockout on Viessmann. Nothing useful can be said about 'E3 on a boiler' without the brand.

Up to: Lockout — engineer likely 6 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 E3 means	Severity	DIY?
AAlpha	The E3 fault code on Alpha boilers indicates that the boiler has detected a problem with water circulation — either the water is not moving through the system adequately, or heat is… Full guide →	Lockout — engineer likely	Some checks
BBaxi	The E3 fault code on a Baxi boiler indicates a problem with the Printed Circuit Board (PCB) — the electronic control unit that acts as the central hub of your boiler's operation. Full guide →	Lockout — engineer likely	No
FFirebird	The E3 code on a Firebird boiler signals a problem with the flue thermistor — a small temperature sensor positioned in the flue outlet that monitors the temperature of exhaust gases… Full guide →	Engineer may be needed	No
PProtherm	The E3 code appears on Protherm boilers when the electronic control unit detects a problem with the domestic hot water (DHW) NTC temperature sensor. Full guide →	Lockout — engineer likely	No
RRemeha	The E3 code appears on Remeha boilers (including the Avanta range) when the boiler detects a problem with the control unit or identifies an earth fault on the electrical supply. Full guide →	Lockout — engineer likely	No
VViessmann	The E3 fault code on Viessmann boilers signals that the burner has entered a fault or lockout state, but the precise cause depends on your specific model. Full guide →	Lockout — engineer likely	Some checks

E3 brand by brand

Alpha E3

The E3 fault code on Alpha boilers indicates that the boiler has detected a problem with water circulation — either the water is not moving through the system adequately, or heat is building up faster than it can be dissipated. Both scenarios point to the same root issue: hot water is not circulating properly around the system. As a safety measure, the boiler locks out to prevent damage to internal components. You will typically lose heating and hot water until the underlying cause is identified and resolved.

Causes, fixes & costs for Alpha E3 → · All Alpha codes

Baxi E3

The E3 fault code on a Baxi boiler indicates a problem with the Printed Circuit Board (PCB) — the electronic control unit that acts as the central hub of your boiler's operation. When the PCB develops a fault, it can no longer send the correct signals to components such as the pump, fan, igniter, and gas valve, so the boiler shuts down entirely as a safety precaution. This is one of the more serious fault codes a Baxi boiler can display, and in most cases it requires professional diagnosis and repair by a Gas Safe registered engineer. Note: the closely related E12 code also points to a PCB fault on Baxi boilers, so if your display shows E12, the same information and steps on this page apply.

Causes, fixes & costs for Baxi E3 → · All Baxi codes

Firebird E3

The E3 code on a Firebird boiler signals a problem with the flue thermistor — a small temperature sensor positioned in the flue outlet that monitors the temperature of exhaust gases leaving the boiler. When the boiler's control board detects an abnormal reading from this sensor (or no reading at all), it logs E3 and may suspend operation to protect the system. On most Firebird Enviromax condensing models the flue thermistor fault is designated E3; on certain Envirogreen Regular units you may see it listed as E2 instead, so always cross-reference your specific model's documentation.

Causes, fixes & costs for Firebird E3 → · All Firebird codes

Protherm E3

The E3 code appears on Protherm boilers when the electronic control unit detects a problem with the domestic hot water (DHW) NTC temperature sensor. This means the signal coming from that sensor is either absent, outside the expected range, or simply doesn't look credible to the boiler's electronics. To protect the appliance, the boiler locks out and displays E3. Note that E3 is associated with older Protherm series such as the Emerald range. If you own a newer Protherm model — such as the Gepard or Panther — your boiler is more likely to use an 'F' prefix for faults (for example, F4 for a DHW sensor failure). Always cross-reference with your specific model's documentation if you're unsure.

Causes, fixes & costs for Protherm E3 → · All Protherm codes

Remeha E3

The E3 code appears on Remeha boilers (including the Avanta range) when the boiler detects a problem with the control unit or identifies an earth fault on the electrical supply. The boiler will lock out and stop working as a safety measure. In plain terms, the boiler's electronics have either lost confidence in their own operation, found an issue with the power supply earth, or detected an anomaly from one of the temperature sensors. Because an earth fault carries a genuine electrical safety risk, this code should be treated seriously and not repeatedly reset without investigation.

Causes, fixes & costs for Remeha E3 → · All Remeha codes

Viessmann E3

The E3 fault code on Viessmann boilers signals that the burner has entered a fault or lockout state, but the precise cause depends on your specific model. On the Vitodens 200-W, it typically means that heat transfer was insufficient during the boiler's calibration process, causing the temperature limiter to shut the unit down. On the Vitodens 100-W, E3 points to a combustion controller that has been incorrectly adjusted — for example, a problem with the ionisation electrode gap or gas-type calibration. Across certain other Viessmann models, E3 (sometimes alongside E1 and E2) can indicate that the heating water flow rate through the system is too low. In all cases the burner will not fire until the underlying condition is resolved. Always check your model's manual to confirm which interpretation applies to your boiler before taking action.

Causes, fixes & costs for Viessmann E3 → · All Viessmann codes

Frequently asked questions

Is the E3 fault code the same on every boiler?

No. E3 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 E3 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a E3 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. For oil boilers, an OFTEC-registered technician. 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.